Nutrient Inputs, Phytoplankton Response, and CO2 Variations in a Semi-Enclosed Subtropical Embayment, Kaneohe Bay, Hawaii

The marine shelf areas in subtropical and tropical regions represent only 35% of the total shelf areas globally, but receive a disproportionately large amount of water (65%) and sediment (58%) discharges that enter such environments. Small rivers and/or streams that drain the mountainous areas in these climatic zones deliver the majority of the sediment and nutrient inputs to these narrow shelf environments; such inputs often occur as discrete, episodic introductions associated with storm events. To gain insight into the linked biogeochemical behavior of subtropical/tropical mountainous watershed-coastal ocean ecosystems, this work describes the use of a buoy system to monitor autonomously water quality responses to land-derived nutrient inputs and physical forcing associated with local storm events in the coastal ocean of southern Kaneohe Bay, Oahu, Hawaii, USA. The data represent 2.5 years of near-real time observations at a fixed station, collected concurrently with spatially distributed synoptic sampling over larger sections of Kaneohe Bay. Storm events cause most of the fluvial nutrient, particulate, and dissolved organic carbon inputs to Kaneohe Bay. Nutrient loadings from direct rainfall and/or terrestrial runoff produce an immediate increase in the N:P ratio of bay waters up to values of 48 and drive phytoplankton biomass growth. Rapid uptake of such nutrient subsidies by phytoplankton causes rapid declines of N levels, return to N-limited conditions, and subsequent decline of phytoplankton biomass over timescales ranging from a few days to several weeks, depending on conditions and proximity to the sources of runoff. The enhanced productivity may promote the drawing down of pCO₂ and lowering of surface water column carbonate saturation states, and in some events, a temporary shift from N to P limitation. The productivity-driven CO₂ drawdown may temporarily lead to air-to-sea transfer of atmospheric CO₂ in a system that is on an annual basis a source of CO₂ to the atmosphere due to calcification and perhaps heterotrophy. Storms may also strongly affect proximal coastal zone pCO₂ and hence carbonate saturation state due to river runoff flushing out high pCO₂ soil and ground waters. Mixing of the CO₂-charged water with seawater causes a salting out effect that releases CO₂ to the atmosphere. Many subtropical and tropical systems throughout the Pacific region are similar to Kaneohe Bay, and our work provides an important indication of the variability and range of CO₂ dynamics that are likely to exist elsewhere. Such variability must be taken into account in any analysis of the direction and magnitude of the air?Csea CO₂ exchange for the integrated coastal ocean, proximal and distal. It cannot be overemphasized that this research illustrates several examples of how high frequency sampling by a moored autonomous system can provide details about ecosystem responses to stochastic atmospheric forcing that are commonly missed by traditional synoptic observational approaches. Finally, the work exemplifies the utility of combining synoptic sampling and real-time autonomous observations to elucidate the biogeochemical and physical responses of coastal subtropical/tropical coral reef ecosystems to climatic perturbations.     

Scales of nutrient-limited phytoplankton productivity in Chesapeake Bay

The scales on which phytoplankton biomass vary in response to variable nutrient inputs depend on the nutrient status of the plankton community and on the capacity of consumers to respond to increases in phytoplankton productivity. Overenrichment and associated declines in water quality occur when phytoplankton growth rate becomes nutrient-saturated, the production and consumption of phytoplankton biomass become uncoupled in time and space, and phytoplankton biomass becomes high and varies on scales longer than phytoplankton generation times. In Chesapeake Bay, phytoplankton growth rates appear to be limited by dissolved inorganic phosphorus (DIP) during spring when biomass reaches its annual maximum and by dissolved inorganic nitrogen (DIN) during summer when phytoplankton growth rates are highest. However, despite high inputs of DIN and dissolved silicate (DSi) relative to DIP (molar ratios of N∶P and Si∶P>100), seasonal accumulations of phytoplankton biomass within the salt-intruded-reach of the bay appear to be limited by riverine DIN supply while the magnitude of the spring diatom bloom is governed by DSi supply. Seasonal imbalances between biomass production and consumption lead to massive accumulations of phytoplankton biomass (often>1,000 mg Chl-a m^?2) during spring, to spring-summer oxygen depletion (summer bottom water <20% saturation), and to exceptionally high levels of annual phytoplankton production (>400 g m^?2 yr^?1). Nitrogen-dependent seasonal accumulations of phytoplankton biomass and annual production occur as a consequence of differences in the rates and pathways of nitrogen and phosphorus cycling within the bay and underscore the importance of controlling nitrogen inputs to the mesohaline and lower reaches of the bay.     

Prehistoric nutrient inputs and productivity in Narragansett Bay

Calculations by others of the preindustrial deposition of inorganic nitrogen from the atmosphere in the area of Narragansett Bay compared with recent measurements suggest that this flux has increased almost 15 times over natural background. On the basis of modern studies of the export of nitrogen and phosphorus from temperate forests, the prehistoric watershed also probably contributed very little reactive N or P to the bay. New information from undisturbed old-growth forests suggests that most of the N that was exported from the watershed was probably associated with refractory dissolved organic matter and thus contributed little to the fertility of the bay. The largest source of reactive dissolved inorganic nitrogen (DIN) and phosphorus (DIP) for Narragansett Bay under prehistoric conditions was the coastal ocean water entrained in the bay in estuarine circulation. The total input of DIN to this estuary has increased about five-fold and the input of total DIP has approximately doubled as a result of human activities. Recent ecosystem-level experiments using large (13 m³, 5 m deep) mesocosms designed as living models of Narragansett Bay showed that the primary production of phytoplankton in the bay is limited by the supply of DIN and that annual phytoplankton production is strongly correlated with the rate of input of DIN. The relationship between DIN input and annual phytoplankton production in the mesocosms is consistent with observations published by others working in 10 different natural marine systems, and a functional regression of the field and experimental data provides a tool to calculate the rate of prehistoric phytoplankton production that would have been associated with the prehistoric DIN input estimates. The result of this calculation suggests that phytoplankton production in the bay has approximately doubled (from about 130 g C m^?2 yr^?1 to 290 g C m^?2 yr^?1 for a baywide average) since the time of European contact. It also seems likely that seagrasses and macroalgae once made a much larger contribution to total system production than they do today.     

Fluvial Fluxes of Water, Suspended Particulate Matter, and Nutrients and Potential Impacts on Tropical Coastal Water Biogeochemistry: Oahu, Hawai‘i

Baseflow and storm runoff fluxes of water, suspended particulate matter (SPM), and nutrients (N and P) were assessed in conservation, urban, and agricultural streams discharging to coastal waters around the tropical island of Oahu, Hawai‘i. Despite unusually low storm frequency and intensity during the study, storms accounted for 8–77% (median 30%) of discharge, 57–99% (median 93%) of SPM fluxes, 11–79% (median 36%) of dissolved nutrient fluxes and 52–99% (median 85%) of particulate nutrient fluxes to coastal waters. Fluvial nutrient concentrations varied with hydrologic conditions and land use; land use also affected water and particulate fluxes at some sites. Reactive dissolved N:P ratios typically were ≥16 (the ‘Redfield ratio’ for marine phytoplankton), indicating that inputs could support new production by coastal phytoplankton, but uptake of dissolved nutrients is probably inefficient due to rapid dilution and export of fluvial dissolved inputs. Particulate N and P fluxes were similar to or larger than dissolved fluxes at all sites (median 49% of total nitrogen, range 22–82%; median 69% of total phosphorus, range 49–93%). Impacts of particulate nutrients on coastal ecosystems will depend on how efficiently SPM is retained in nearshore areas, and on the timing and degree of transformation to reactive dissolved forms. Nevertheless, the magnitude of particulate nutrient fluxes suggests that they represent a significant nutrient source for many coastal ecosystems over relatively long time scales (weeks–years), and that reductions in particulate nutrient loading actually may have negative impacts on some coastal ecosystems.     

Dissolved Inorganic Nitrogen,Soluble Reactive Phosphorous,and Microbial Pollutant Loading from Tropical Rural Watersheds in Hawai'i to the Coastal Ocean During Non-Storm Conditions

This study quantifies dissolved inorganic nitrogen (DIN), soluble reactive phosphorous (SRP), and microbial pollutant inputs to a tropical embayment, Hanalei Bay, Kaua'i, Hawai'i from rural watersheds during two field excursions during non-storm conditions. We employ land cover analysis and a suite of nucleic acid fecal source tracking markers (host-specific Bacteroidales and human enterovirus) to identify sources of pollutants to the bay. The highest concentrations of DIN and SRP are in streams draining watersheds with large areas of cultivated land, suggesting fertilizer is a source of these nutrients to the streams and coastal waters. Pollutant areal loading correlates with the fractions of urban and cultivated land cover. Microbial source tracking indicates the presence of human, pig, and ruminant feces in the streams. This work provides preliminary evidence that human development affects loading of DIN, SRP, and microbial pollutants to tropical coastal waters; further study is needed to confirm this. Additionally, results point to a mix of microbial pollutant sources.     

Impacts of SW Monsoon on Phytoplankton Community Structure Along the Western Coastal BOB: an HPLC Approach

The major Indian rivers bring significant amount of freshwater along with inorganic nutrients and sediment load in to the northern Bay of Bengal (BOB) during the southwest monsoon (SWM); the southern bay does not experience equal freshening. This contrasting pattern may considerably impact the physicochemical features and phytoplankton community composition in this bay and was investigated during a coastal cruise during the SWM covering eight river plumes from both northern and southern bay; phytoplankton pigments and physicochemical parameters were analysed from different depths (0, 10, 25, and 50 m). Significant freshening, stratification and warmer waters were noticed in the northern bay relative to its southern part. Phytoplankton pigment analysis and diagnostic pigment-based size class analysis revealed the dominance of microphytoplankton (mainly diatoms) in the northern bay and were mostly confined to the surface waters. Their abundance was positively correlated with dissolved silicate (DSi) concentrations and inversely with salinity. Nanophytoplankton and picophytoplankton (prymnesiophytes, chrysophytes and cyanophytes) were mostly noticed in the subsurface waters and dominated the southern bay. This finding suggests that the dominance of microphytoplankton in the northern bay may significantly contribute to higher particle flux which has been reported earlier. Therefore, any modification in future river discharge, which is in turn related to the intensity of Indian summer monsoon, will alter the phytoplankton community structure in the coastal BOB and may be further cascaded to the other vital ecosystem components like fisheries resources, organic carbon export flux and benthic production.     

Marsh-water column interactions in two Louisiana estuaries. II. Nutrient dynamics

The exchange of dissolved nutrients between marshes and the inundating water column was measured using throughflow marsh flumes built, in two microtidal Louisiana estuaries: the Barataria Basin estuary and Fourleague Bay. The flumes were sampled between September 1986 and April 1988, coincident with an extended period of low sea level on the Louisiana coast. The Barataria Basin estuary is in the later, deteriorating stage of the deltaic cycle, characterized by low freshwater inputs and subsiding marshes. Both brackish and saline marshes supplied dissolved organic nitrogen (DON), inorganic nitrogen (ammonium + nitrate + nitrite = DIN), dissolved organic carbon (DOC), and total nitrogen (as total Kjeldahl nitrogen = TKN) to the water column. The export of DIN is probably related to the N accumulated in earlier stages of deltaic development and released as these marshes deteriorate. Coastal brackish marshes of Fourleague, Bay, part of an accreting marsh system in an early, developmental stage of the deltaic cycle, exported TKN to the open water estuary in all samplings. This marsh apparently acted as a short-term buffer of DIN by taking up NH₄ ⁺ in spring, when baywide concentrations were high, and supplying DIN to the estuary in summer and fall, when concentrations, in the bay were lower. Differences in phosphorus (P), DOC, and DON fluxes between these two estuaries were also observed. The Fourleague Bay site exported soluble reactive phosphorus (SRP) and total phosphorus (TP) and imported DOC. This P export may be related to remobilization of sediment-bound riverine P by the reducing, soils of the marshes. Fluxes of SRP at the Barataria Basin sites were variable and low while DOC was imported. Most imports of dissolved nutrients were correlated with higher upstream [source] concentrations, and flux rates were fairly consistent throughout the tide. Dissolved nutrient exports, did not correlate with upstream concentrations, though, and in many cases the flux was dominated by early, flood tide nutrient release. This pulsed behavior may be caused by rapid diffusion from the sediments early in the tidal cycle, when the sediment-water concentration gradient is largest. Interestuary differences were also seen in particulate organic matter fluxes, as the Fourleague Bay marsh exported POC and PON during all samplings while Barataria Basin imported these nutrients. In general, the magnitude and direction of nutrient exchanges in Louisiana marshes, seem to reflect the deltaic successional stage of the estuary.     

Coupling Between the Coastal Ocean and Yaquina Bay,Oregon: Importance of Oceanic Inputs Relative to Other Nitrogen Sources

Understanding of the role of oceanic input in nutrient loadings is important for understanding nutrient and phytoplankton dynamics in estuaries adjacent to coastal upwelling regions as well as determining the natural background conditions. We examined the nitrogen sources to Yaquina Estuary (Oregon, USA) as well as the relationships between physical forcing and gross oceanic input of nutrients and phytoplankton. The ocean is the dominant source of dissolved inorganic nitrogen (DIN) and phosphate to the lower portion of Yaquina Bay during the dry season (May through October). During this time interval, high levels of dissolved inorganic nitrogen (primarily in the form of nitrate) and phosphate entering the estuary lag upwelling favorable winds by 2 days. The nitrate and phosphate levels entering the bay associated with coastal upwelling are correlated with the wind stress integrated over times scales of 4–6 days. In addition, there is a significant import of chlorophyll a to the bay from the coastal ocean region, particularly during July and August. Variations in flood-tide chlorophyll a lag upwelling favorable winds by 6 days, suggesting that it takes this amount of time for phytoplankton to utilize the recently upwelled nitrogen and be transported across the shelf into the estuary. Variations in water properties determined by ocean conditions propagate approximately 11–13 km into the estuary. Comparison of nitrogen sources to Yaquina Bay shows that the ocean is the dominant source during the dry season (May to October) and the river is the dominant source during the wet season with watershed nitrogen inputs primarily associated with nitrogen fixation on forest lands.     

Nutrient biogeochemistry in an upwelling-influenced estuary of the Pacific northwest (Tillamook Bay,Oregon, USA)

Tillamook Bay, Oregon, is a drowned river estuary that receives freshwater input from 5 rivers and exchanges ocean water through a single channel. Similar to other western United States estuaries, the bay exhibits a strong seasonal change in river discharge in which there is a pronounced winter maximum and summer minimum in precipitation and runoff. The behavior of major inorganic nutrients (phosphorus, nitrogen, and silica) within the watershed is examined over seasonal cycles and under a range of river discharge conditions for October 1997–December 1999. Monthly and seasonal sampling stations include transects extending from the mouth of each river to the mouth of the estuary as well as 6–10 sites upstream along each of the 5 major rivers. Few studies have examined nutrient cycling in Pacific Northwest estuaries. This study evaluates the distributions of inorganic nutrients to understand the net processes occurring within this estuary. Based upon this approach, we hypothesize that nutrient behavior in the Tillamook Bay estuary can be explained by two dominant factors: freshwater flushing time and biological uptake and regeneration. Superimposed on these two processes is seasonal variability in nutrient concentrations of coastal waters via upwelling. Freshwater flushing time determines the amount of time for the uptake of nutrients by phytoplankton, for exchange with suspended particles, and for interaction with the sediments. Seasonal coastal upwelling controls the timing and extent of oceanic delivery of nutrients to the estuary. We suggest that benthic regeneration of nutrients is also an important process within the estuary occurring seasonally according to the flushing characteristics of the estuary. Silicic acid, nitrate, and NH₄ ⁺ supply to the bay appears to be dominated by riverine input. PO₄ ⁻³ supply is dominated by river input during periods of high river flow (winter months) with oceanic input via upwelling and tidal exchange important during other times (spring, summer, and fall months). Departures from conservative mixing indicate that internal estuarine sources of dissolved inorganic phosphorus and nitrogen are also significant over an annual cycle.     

Multiple Factors driving Variability of CO2 Exchange Between the Ocean and Atmosphere in a Tropical Coral Reef Environment

In this paper, we present the results of the first automated continuous multi-year high temporal frequency study of CO₂ dynamics in a coastal coral reef ecosystem. The data cover 2.5?years of nearly continuous operation of the CRIMP-CO₂ buoy spanning particularly wet and dry seasons in southern Kaneohe Bay, a semi-enclosed tropical coral reef ecosystem in Hawaii. We interpret our observational results in the context of how rapidly changing physical and biogeochemical conditions affect the pCO₂ of surface waters and the magnitude and direction of air–sea exchange of CO₂. Local climatic forcing strongly affects the biogeochemistry, water column properties, and gas exchange between the ocean and atmosphere in Kaneohe Bay. Rainfall driven by trade winds and other localized storms generates pulses of nutrient-rich water, which exert a strong control on primary productivity and impact carbon cycling in the water column of the bay. The “La Ni?a” winter of 2005–2006 was one of the wettest winters in Hawaii in 30?years and contrasted sharply with preceding and subsequent drier winter seasons. In addition, short-term variability in physical forcing adds complexity and helps drive the response of the CO₂–carbonic acid system of the bay. Freshwater pulses to Kaneohe Bay provide nutrient subsidies to bay waters, relieving the normal nitrogen limitation of this system and driving phytoplankton productivity. Seawater pCO₂ responds to the blooms as well as to physical forcing mechanisms, leading to a relatively wide range of pCO₂ in seawater from about 250 to 650?μatm, depending on conditions. Large drawdowns in pCO₂ following storms occasionally cause bay waters to switch from being a source of CO₂ to the atmosphere to being a sink. Yet, during our study period, the southern sector of Kaneohe Bay remained a net source of CO₂ to the atmosphere on an annualized basis. The integrated net annual flux of CO₂ from the bay to the atmosphere varied between years by a factor of more than two and was lower during the wet “La Ni?a” year, than during the following year. Over the study period, the net annualized flux was 1.80?mol?C?m^?2?year^?1. Our CO₂ flux estimates are consistent with prior synoptic work in Kaneohe Bay and with estimates in other tropical coral reef ecosystems studied to date. The high degree of climatological, physical, and biogeochemical variability observed in this study suggests that automated high-frequency observations are needed to capture the short-, intermediate-, and long-term variability of CO₂ and other properties of these highly dynamic coastal coral reef ecosystems.     

Phosphorus and nitrogen inputs to Florida Bay: The importance of the everglades watershed

A large environmental restoration project designed to improve the hydrological conditions of the Florida Everglades and increase freshwater flow to Florida Bay is underway. Here we explore how changing freshwater inflow to the southern Everglades is likely to change the input of nutrients to Florida Bay. We calculated annual inputs of water, total phosphorus (TP), total nitrogen (TN), and dissolved inorganic nitrogen (DIN) to Everglades National Park (ENP) since the early 1980s. We also examined changes in these nutrient concentrations along transects through the wetland to Florida Bay and the Gulf of Mexico. We found that the interannual variability of the water discharge into ENP greatly exceeded the interannual variability of flow-weighted mean nutrient concentrations in this water. Nutrient inputs to ENP were largely determined by discharge volume. These inputs were high in TN and low in TP; for two ENP watersheds TN averaged 1.5 mg l^?1 (0.11 mM) and 0.9 mg l^?1 (0.06 mM) and TP averaged 15 μg l^?1 (0.47 μM) and 9 μg l^?1 (0.28 μM). Both TP and DIN that flowed into ENP wetlands were rapidly removed from the water. Over a 3-km section of Taylor Slough, TP decreased from a flow-weighted mean of 11.6 μg l^?1 (0.37 μM) (0.20 μM) and DIN decreased from 240 μg l^?1 (17μM) to 36 μ l^?1 (2.6 μM). In contrast, TN, which was generally 95% organic N, changed little as it passed through the wetland. This resulted in molar TN:TP ratios exceeding 400 in the wetland. Decreases in TN concentrations only occurred in areas with relatively high P availability, such as the wetlands to the north of ENP and in the mangrove streams of western ENP. Increasing freshwater flow to Florida Bay in an effort to restore the Everglades and Florida Bay ecosystems is thus not likely to increase P inputs from the freshwater Everglades but is likely to increase TN inputs. Based on a nutrient budget of Florida Bay, both N and P inputs from the Gulf of Mexico greatly exceed inputs from the Everglades, as well as inputs from the atmosphere and the Florida Keys. We estimate that the freshwater Everglades contribute <3% of all P inputs and <12% of all N inputs to the bay. Evaluating the effect of ecosystem restoration efforts on Florida Bay requires greater understanding of the interactions of the bay with the Gulf of Mexico and adjacent mangrove ecosystems.     

Spatial and Temporal Variability of Benthic Oxygen Demand and Nutrient Regeneration in an Anthropogenically Impacted New England Estuary

Strong benthic–pelagic coupling is an important characteristic of shallow coastal marine ecosystems. Building upon a rich history of benthic metabolism data, we measured oxygen uptake and nutrient fluxes across the sediment–water interface along a gradient of water column primary production in Narragansett Bay, RI (USA). Despite the strong gradients seen in water column production, sediment oxygen demand (SOD) and benthic nutrient fluxes did not exhibit a clear spatial pattern. Some of our sites had been studied in the 1970s and 1980s and thus allowed historical comparison. At these sites, we found that SOD and benthic fluxes have not changed uniformly throughout Narragansett Bay. In the uppermost portion of the bay, the Providence River Estuary, we observed a significant decrease in dissolved inorganic phosphorus fluxes which we attribute to management interventions. At another upper bay site, we observed significant declines in SOD and dissolved inorganic nitrogen fluxes which may be linked to climate-induced decreases in water column primary production and shifts in bloom phenology. In the 1970s, benthic nutrient regeneration supplied 50% to over 200% of the N and P needed to support primary production by phytoplankton. Summer nutrient regeneration in the Providence River Estuary and Upper bay now may only supply some 5–30% of the N and 3–20% of the P phytoplankton demand.     

Watershed Export Events and Ecosystem Responses in the Mission–Aransas National Estuarine Research Reserve, South Texas

While inputs from land are recognized as important resources supporting production in estuaries, the role that storm events play in resource delivery and the response of estuarine systems to pulsed inputs are less well appreciated. Temporal variations in (1) watershed export via the Mission and Aransas rivers and (2) nutrient and organic matter dynamics in Copano Bay were examined in the Mission?CAransas National Estuarine Research Reserve, south Texas. Inorganic nutrient, dissolved organic matter (DOM), and particulate organic matter (POM) concentrations in the rivers varied substantially with discharge, as did carbon and nitrogen stable isotope ratios of POM. Accounting for these variations was critical for calculating robust watershed export estimates. In Copano Bay, soluble reactive phosphorus, POM, and DOM remained elevated for several months following major runoff events, whereas inorganic nitrogen was rapidly depleted. Chlorophyll-a concentrations and POC-??¹³C in Copano Bay showed that increased POM concentrations were linked to enhanced in situ production.     

External nutrient sources,internal nutrient pools,and phytoplankton production in Chesapeake Bay

External nutrient loadings, internal nutrient pools, and phytoplankton production were examined for three major subsystems of the Chesapeake Bay Estuary—the upper Mainstem, the Patuxent Estuary, and the 01 Potomac Estuary—during 1985–1989. The atomic nitrogen to phosphorus ratios (TN:TP) of total loads to the 01 Mainstem, Patuxent, and the Potomac were 51, 29 and 35, respectively. Most of these loads entered at the head of the estuaries from riverine sources and major wastewater treatment plants. Approximately 7–16% for the nitrogen load entered the head of each estuary as particulate matter in contrast to 48–69% for phosphorus. This difference is hypothesized to favor a greater loss of phosphorus than nitrogen through sedimentation and burial. This process could be important in driving estuarine nitrogen to phosphorus ratios above those of inputs. Water column TN: TP ratios in the tidal fresh, oligohaline, and mesohaline salinity zones of each estuary ranged from 56 to 82 in the Mainstem, 27 to 48 in the Patuxent, and 72 to 126 in the Potomac. A major storm event in the Potomac watershed was shown to greatly increase the particulate fraction of nitrogen and phosphorus and lower the TN:TP in the river-borne loads. The load during the month that contained this storm (November 1985) accounted for 11% of the nitrogen and 31% of the phosphorus that was delivered to the estuary by the Potomac River during the entire 60-month period examined here. Within the Mainstem estuary, salinity dilution plots revealed strong net sources of ammonium and phosphate in the oligohaline to upper mesohaline region, indicating that these areas were sites of considerable internal recycling of nutrients to surface waters. The sedimentation of particulate nutrient loads in the upper reaches of the estuary is probably a major source of these recycled nutrients. A net sink of nitrate was indicated during summer. A combination of inputs and these internal recycling processes caused dissolved inorganic N to P ratios to approach 16:1 in the mesohaline zone of the Mainstem during late summer; this ratio was much higher at other times and in the lower salinity zones. Phytoplankton biomass in the mesohaline Mainstem reached a peak in spring and was relatively constant throughout the other seasons. Productivity was highest in spring and summer, accounting for approximately 33% and 44%, respectively, of the total annual productivity in this region. In the Patuxent and Potomac, the TN:TP ratios of external loads documented here are 2–4 times higher than those observed over the previous two decades. These changes are attributed to point-source phosphorus controls and the likelihood that nitrogen-rich nonpoint source inputs, including contributions from the atmosphere, have increased. These higher N:P ratios relative to Redfield proportions (16:1) now suggest a greater overall potential for phosphorus-limitation rather than nitrogen-limitation of phytoplankton in the areas studied.     

Sources of nitrogen and phosphorus to Northern San Francisco Bay

We studied nutrient sources to the Sacramento River and Suisun Bay (northern San Francisco Bay) and the influence which these sources have on the distributions of dissolved inorganic nitrogen (DIN) and dissolved reactive phosphorus (DRP) in the river and bay. We found that agricultural return flow drains and a municipal wastewater treatment plant were the largest sources of nutrients to the river during low river flow. The Sutter and Colusa agricultural drains contributed about 70% of the transport of DIN and DRP by the river above Sacramento (about 20% of the total transport by the river) between August 8 and September 26, 1985. Further downstream, the Sacramento Regional Wastewater Treatment Plant discharged DIN and DRP at rates that were roughly 70% of total DIN and DRP transport by the river at that time. Concentrations at Rio Vista on the tidal river below the Sacramento plant and at the head of the estuary were related to the reciprocals of the river flows, indicating the importance of dilution of the Sacramento waste by river flows. During very dry years, elevated DIN and DRP concentrations were observed in Suisun Bay. We used a steady-state, one-dimensional, single-compartment box model of the bay, incorporating terms for advection, exchange, and waste input, to calculate a residual rate for all processes not included in the model. We found that the residual for DIN was related to concentrations of chlorophylla (Chla). The residual for DRP was also related to Chla at high concentrations of Chla, but showed significant losses of DRP at low Chla concentrations. These losses were typically equivalent to about 80% of the wastewater input rate.     

Long-Term and Seasonal Changes in Nutrients,Phytoplankton Biomass,and Dissolved Oxygen in Deep Bay,Hong Kong

Deep Bay is a semienclosed bay that receives sewage from Shenzhen, a fast-growing city in China. NH₄ is the main N component of the sewage (>50% of total N) in the inner bay, and a twofold increase in NH₄ and PO₄ concentrations is attributed to increased sewage loading over the 21-year period (1986–2006). During this time series, the maximum annual average NH₄ and PO₄ concentrations exceeded 500 and 39 μM, respectively. The inner bay (Stns DM1 and DM2) has a long residence time and very high nutrient loads and yet much lower phytoplankton biomass (chlorophyll (Chl) <10 μg L⁻¹ except for Jan, July, and Aug) and few severe long-term hypoxic events (dissolved oxygen (DO) generally >2 mg L⁻¹) than expected. Because it is shallow (~2 m), phytoplankton growth is likely limited by light due to mixing and suspended sediments, as well as by ammonium toxicity, and biomass accumulation is reduced by grazing, which may reduce the occurrence of hypoxia. Since nutrients were not limiting in the inner bay, the significant long-term increase in Chl a (0.52–0.57 μg L⁻¹ year⁻¹) was attributed to climatic effects in which the significant increase in rainfall (11 mm year⁻¹) decreased salinity, increased stratification, and improved water stability. The outer bay (DM3 to DM5) has a high flushing rate (0.2 day⁻¹), is deeper (3 to 5 m), and has summer stratification, yet there are few large algal blooms and hypoxic events since dilution by the Pearl River discharge in summer, and the invasion of coastal water in winter is likely greater than the phytoplankton growth rate. A significant long-term increase in NO₃ (0.45–0.94 μM year⁻¹) occurred in the outer bay, but no increasing trend was observed for SiO₄ or PO₄, and these long-term trends in NO₃, PO₄, and SiO₄ in the outer bay agreed with those long-term trends in the Pearl River discharge. Dissolved inorganic nitrogen (DIN) has approximately doubled from 35–62 to 68–107 μM in the outer bay during the last two decades, and consequently DIN to PO₄ molar ratios have also increased over twofold since there was no change in PO₄. The rapid increase in salinity and DO and the decrease in nutrients and suspended solids from the inner to the outer bay suggest that the sewage effluent from the inner bay is rapidly diluted and appears to have a limited effect on the phytoplankton of the adjacent waters beyond Deep Bay. Therefore, physical processes play a key role in reducing the risk of algal blooms and hypoxic events in Deep Bay.     

Estuarine Phytoplankton Responses to Hurricanes and Tropical Storms with Different Characteristics (Trajectory,Rainfall, Winds)

We examined the short-term (<1 month post-storm) impact of storms [Tropical Storm (TS) Helene in 2000, Hurricane (H) Isabel in 2003, H Alex, Tropical Depression (TD) Bonnie and TS Charley in 2004] varying in their trajectory, wind and rainfall characteristics, on water column structure, nutrients, and phytoplankton biomass in North Carolina’s Neuse R. Estuary (NRE). Data are presented from two sampling programs, ModMon (biweekly) and FerryMon (measurements made every 3 min daily). Helene’s winds mixed the previously stratified water column, delivering sediment-bound nutrients to the euphotic zone, and localized freshwater input from Helene was also evident. Mean chlorophyll a concentrations in the mesohaline portion of the NRE, where N was strongly limiting before the storm (molar DIN:DIP < 1), more than doubled after the storm. Unlike with Helene, the water column was well mixed before passage of Isabel, and nutrient concentrations were high. As a result, minimal impact on phytoplankton biomass was detected despite Isabel’s high winds and significant freshwater input. In fact, conditions became less favorable for phytoplankton growth after the storm. Alex was fast moving and relatively small, but its winds were sufficient to mix the water column. Although data from ModMon suggest that chlorophyll a was only slightly higher after passage of Alex, FerryMon detected an ephemeral bloom that was missed by ModMon. Overall, these results suggest that relatively small tropical storms and hurricanes can lead to significant increases in phytoplankton biomass. However, the phytoplankton response depends on both the characteristics of a particular storm and the physical–chemical conditions of the water column before storm passage. Finally, the ephemeral bloom that developed as a result of Alex, the strong response of phytoplankton in the mesohaline portion of the estuary to nutrient inputs, and their patchiness on several other occasions suggests that storms may create “hot spots” for trophic transfer and biogeochemical dynamics in estuaries. Adaptive sampling is necessary to capture these features and to fully understand the impact of perturbations such as storms on estuarine ecosystem functioning.     

Impact of storm runoff from tropical watersheds on coastal water quality and productivity

Storm runoff in the steep watersheds in Hawaii leads to sediment and freshwater pulses to coastal waters that quickly affect nearshore water quality. This is particularly true in semi-enclosed embayments, such as Kaneohe Bay, Oahu, where water has a relatively long residence time compared to more open coastal areas of the islands. In this paper the authors discuss water quality and productivity in Kaneohe Bay after back-to-back rain events in late November and early December 2003, following a particularly dry summer. The short-term biogeochemical response of coastal waters and the ecosystem to runoff and physical forcing was evaluated through a combination of continuous in situ measurements and adaptive synoptic sampling carried out on a variety of temporal and spatial scales.     

Nutrient cycling in the sub-tropical Brunswick estuary,Australia

A combination of mixing plots, one-dimensional salt balance modelling, nutrient loading budgets, and benthic flux measurements were used to assess nutrient cycling pathways in the enriched sub-tropical Brunswick estuary during different freshwater flows. A simple model accounting for freshwater residence times and nutrient availability was found to be a good predictor of phytoplankton biomass along the estuary, and suggested that biomass accumulation may become nutrient-limited during low flows and that recycling within the water column is important during blooms. Dissolved inorganic nitrogen (DIN) cycling budgets were constructed for the estuary during different freshwater flows accounting for all major inputs (catchment, sewage, and urban) to the estuary. Internal cycling due to phytoplankton uptake (based on measured biomass) and sediment-water fluxes (based on measured rates in each estuarine reach) was considered. Four different nutrient cycling states were identified during the study. In high flow, freshwater residence times are less than 1 d, internal cycling processes are bypassed and virtually all dissolved, and most particulate, nutrients are delivered to the continental shelf. During the growth phase of a phytoplankton bloom enhanced recycling occurs as residence times increase sufficiently to allow biomass accumulation. Remineralization of phytoplankton detritus during this phase can supply up to 50% of phytoplankton DIN demands. In post-bloom conditions, DIN uptake by phytoplankton decreases in the autumn wet season when biomass doubling times begin to exceed residence times. OM supply to the sediments diminishes and the benthos becomes nutrient-limited, resulting in DIN uptake by the sediments. As flows decrease further in the dry season, there is tight recycling and phytoplankton blooms, and uptake by the sediments can account for the entire DIN loading to the estuary resulting in complete removal of DIN from the water column. The ocean is a potentially important source of DIN to the estuary at this time. The results of the DIN cycling budgets compared favorably with mixing plots of DIN at each time. The results suggest that a combination of different approaches may be useful in developing a more comprehensive understanding of nutrient cycling behavior and the effects of nutrient enrichment in estuaries.     

Nutrient limitation of heterotrophic bacteria in Florida Bay

We examined heterotrophic bacterial nutrient limitation at four sites in Florida Bay, U. S. in summer 1994 and winter 1995. Bacterial growth and biomass production in this system were most limited by inorganic phosphorus (P) in the eastern and southern regions of the bay. Nutrient additions stimulated productivity and biomass accumulation mostly in summer. The magnitude of growth responses (thymidine incorporation) to nutrient additions was nearly an order of magnitude less in winter than summer. Biomass-normalized alkaline phosphatase activity in the northeast and south-central region was 5–20 times greater than in the northwest and north-central regions, suggesting that P is most limiting to planktonic growth in those areas. Chlorophyll levels were higher in the northwest and north-central regions and P-uptake into particles >1 μm, primarily phytoplankton, was also higher in these regions. Consistent with these observations, others have observed that P is advected into the bay primarily in the northwestern region. Abundant seagrasses in Florida Bay may promote heterotrophic bacterial production relative to phytoplankton production by releasing dissolved organic carbon that makes bacteria more competitive for limiting quantities of inorganic phosphate, especially in the eastern bay where turbidity is low, P is most limiting, and light levels reaching the benthic plants are high.