Nutrient flux in a landscape: Effects of coastal land use and terrestrial community mosaic on nutrient transport to coastal waters

Long-term interdisciplinary studies of the Rhode River estuary and its watershed in the mid-Atlantic coastal plain of North America have measured fluxes of nitrogen and phosphorus fractions through the hydrologically-linked ecosystems of this landscape. These ecosystems are upland forest, cropland, and pasture; streamside riparian forests; floodplain swamps; tidal brackish marshes and mudflats; and an estuarine embayment. Croplands discharged far more nitrogen per hectare in runoff than did forests and pastures. However, riparian deciduous hardwood forest bordering the cropland removed over 80 percent of the nitrate and total phosphorus in overland flows and about 85 percent of the nitrate in shallow groundwater drainage from cropland. Nevertheless, nutrient discharges from riparian forests downslope from croplands still exceeded discharges from pastures and other forests. The atomic ratio of nitrogen to phosphorus discharged from the watersheds into the estuary was about 9 for total nutrients and 6 for inorganic nutrient fractions. Such a low N:P ratio would promote nitrogen rather than phosphorus limitation of phytoplankton growth in the estuary. Estuarine tidal marshes trapped particulate nutrients and released dissolved nutrients. Subtidal mudflats in the upper estuary trapped particulate P, released dissolved phosphate, and consumed nitrate. This resulted in a decrease in the ratio of dissolved inorganic N:P in the estuary. However, the upper estuary was a major sink for total phosphorus due to sediment accretion in the subtidal area. Bulk precipitation accounted for 31 percent of the total nongaseous nitrogen influx to the landscape, while farming accounted for 69 percent. Forty-six percent of the total non-gaseous nitrogen influx was removed as farm products, 53 percent either accumulated in the watershed or was lost in gaseous forms, and 1 percent entered the Rhode River. Of the total phosphorus influx to the landscape, 7 percent was from bulk precipitation and 93 percent was from farming. Forty-five percent of the total phosphorus influx was removed as farm products, 48 percent accumulated in the watershed, and 7 percent entered the Rhode River. These nitrogen and phosphorus discharges into the Rhode River, although a small fraction of total loadings to the watershed, were large enough to cause seriously overenriched conditions in the upper estuary.     

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.     

Phytoplankton production and nutrient distributions in a subtropical estuary: Importance of freshwater flow

The relationships between phytoplankton productivity, nutrient distributions, and freshwater flow were examined in a seasonal study conducted in Escambia Bay, Florida, USA, located in the northeastern Gulf of Mexico. Five sites oriented along the salinity gradient were sampled 24 times over the 28-mo period from 1999 to 2001. Water column profiles of temperature and salinity were measured along with surface chlorophyll and surface inorganic nutrient concentrations. Primary productivity was measured at 2 sites on 11 dates, and estimated for the remaining dates and sites using an empirical regression model relating phytoplankton net production to the product of chlorophyll, euphotic zone depth, and daily solar insolation. Freshwater flow into the system varied markedly over the study period with record low flow during 2000, a flood event in March 2001, and subsequent resumption of normal flow. Flushing times ranged from 1 d during the flood to 20 d during the drought. Freshwater input strongly affected surface salinity distributions, nutrient flux, chlorophyll, and primary productivity. The flood caused high turbidity and rapid flushing, severely reducing phytoplankton production and biomass accumulation. Following the flood, phytoplankton biomass and productivity sharply increased. Analysis of nutrient distributions suggested Escambia Bay phytoplankton alternated between phosphorus limitation during normal flow and nitrogen limitation during low flow periods. This study found that Escambia Bay is a moderately productive estuary, with an average annual integrated phytoplankton production rate of 290 g C m⁻² yr⁻¹.     

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.     

Climatic Influences on Autochthonous and Allochthonous Phytoplankton Blooms in a Subtropical Estuary, St. Lucie Estuary, Florida, USA

The St. Lucie Estuary, located on the southeast coast of Florida, provides an example of a subtropical ecosystem where seasonal changes in temperature are modest, but summer storms alter rainfall regimes and external inputs to the estuary from the watershed and Atlantic Ocean. The focus of this study was the response of the phytoplankton community to spatial and temporal shifts in salinity, nutrient concentration, watershed discharges, and water residence times, within the context of temporal patterns in rainfall. From a temporal perspective, both drought and flood conditions negatively impacted phytoplankton biomass potential. Prolonged drought periods were associated with reduced nutrient loads and phytoplankton inputs from the watershed and increased influence of water exchange with the Atlantic Ocean, all of which restrict biomass potential. Conversely, under flood conditions, nutrient loads were elevated, but high freshwater flushing rates in the estuary diminished water residence times and increase salinity variation, thereby restricting the buildup of phytoplankton biomass. An exception to the latter pattern was a large incursion of a cyanobacteria bloom from Lake Okeechobee via the St. Lucie Canal observed in the summer of 2005. From a spatial perspective, regional differences in water residence times, sources of watershed inputs, and the proximity to the Atlantic Ocean influenced the composition and biomass of the phytoplankton community. Long water residence times in the North Fork region of the St. Lucie Estuary provided an environment conducive to the development of blooms of autochthonous origin. Conversely, shorter residence times in the mid-estuary limit autochthonous increases in biomass, but allochthonous sources of biomass can result in bloom concentrations of phytoplankton.     

Mixing behavior of dissolved organic carbon and its potential biological significance in the Pawcatuck River Estuary

The distributions of dissolved organic carbon (DOC), phytoplankton biomass (as measured by in vivo fluorescence), total nitrogen and phosphorus, and light extinction were observed on 10 cruises during 1989 and 1990 in the Pawcatuck River estuary located in southern Rhode Island. In the lower estuary, the distance of peak phytoplankton biomass from the head of the estuary was positively correlated with river discharge while the magnitude of the peak increased with decreasing discharge. High light-extinction appeared to limit the accumulation of biomass in the upper estuary. Variability in light extinction was largely (50%) explained by variation in the concentration of DOC. Salinity versus constituent plots suggested that DOC behaved nonconservatively in the estuary. These observational data indicate that the mixing behavior of DOC in the estuary influences light extinction and thus may limit accumulation of phytoplankton biomass in the upper estuary. This interpretation of observational data was supported by experimental work that demonstrated the significant contribution of DOC to light extinction, and by measurements, of phytoplankton productivity that showed greater light limitation in the upper estuary.     

Nutrient limitation in the lower Housatonic River estuary

One of the most serious threats to freshwater and marine ecosystems is high rates of anthropogenic nutrient loading, particularly nitrogen (N) and phophorus (P). One of the major freshwater sources of nutrients to Long Island Sound (LIS) is the Housatonic River (HR). Current management plans that call for reducing N inputs without reducing P inputs may change the N: P ratio in the water column and the pattern of algal nutrient limitation and species composition in the tidal portion of the river. To assess the current pattern of algal nutrient limitation in the HR estuary, nutrient bioassays were conducted in spring, summer, and fall at 5 sites throughout the tidal portion and adjacent LIS. Diatoms were a dominant taxon at all sites throughout the sampling period. Other seasonally important taxa include cyanobacteria, cryptophytes, and euglenoids. Phytoplankton in LIS were always strongly N limited and were co-limited by P in spring. During low flow (summer), phytoplankton in the lower HR estuary were N limited. Phytoplankton in the middle reaches showed no evidence of N or P limitation and were likely limited by other factors. In spring, phytoplankton in the upper HR estuary were P limited. Periods of N or P limitation were better correlated with periods of lower concentrations of nitrate or phosphate than with differences in N: P ratio. These results suggest that decreases in N concentration could increase the prevalence of N limitation throughout the estuary that in turn may reduce phytoplankton biomass and alter species composition of the phytoplankton.     

Effects of zooplankton grazing on phytoplankton size-structure and biomass in the lower Hudson River estuary

The impact of mesozooplankton (>210 μm, mostly adult copepods and late-stage copepodites) and micrometazoa (64–210 μm, mostly copepod nauplii) on phytoplankton size structure and biomass in the lower Hudson River estuary was investigated using various¹⁴C-labeled algal species as tracers of grazing on natural phytoplankton. During spring and summer, zooplankton grazing pressure, defined as %=mg C ingested m^?2 h^?1/mg C produced m^?2 h^?1 (depth-integrated rates)×100, on total phytoplankton ranged between 0.04% and 1.9% for mesozooplankton and 0.1% and 6.6% for micrometazoa. The greatest grazing impact was measured in fall when 20.2% and 44.6%, respectively, of the total depth-integrated primary production from surface water phytoplankton was grazed. Mesozooplankton exhibited some size-selective grazing on phytoplankton, preferentially grazing the diatomThalassiosira pseudonana over the larger diatomDitylum brightwelli, but this was not found for micrometazoa. Neither zooplankton group grazed on the dinoflagellateAmphidinium sp. We conclude that metazoan zooplankton have a minimal role in controlling total phytoplankton biomass in the lower Hudson River estuary. Differences in the growth coefficients of various phytoplankton size-fractions—not grazing selectivity—may be the predominant factor explaining community size-structure.     

The influence of phytoplankton community composition on primary productivity along the riverine to freshwater tidal continuum in the San Joaquin River,California

Differences in phytoplankton community composition along a riverine to, freshwater tidal continuum was an important factor affecting the primary productivity and quantity of phytoplankton biomass available to the San Francisco Estuary food web downstream. The relative contribution of riverine and freshwater tidal phytoplankton was determined using measurements of primary productivity, respiration, and phytoplankton species composition along a riverine to freshwater tidal gradient in the San Joaquin River, one of two major rivers that flow into, the San Francisco Estuary. Chla-specific net primary productivity was greater in the freshwater tidal habitat and was correlated with both a higher growth efficiency and maximum growth potential compared with the river upstream. Cluster analysis indicated these differences in growth parameters were associated with differences in species composition, with greater percent diatom and green algal species biomass upstream and flagellate biomass downstream. Correlation between the chla specific net productivity and phytoplankton species composition suggested the downstream shift from riverine diatom and green algal species to flagellate species contributed to the seaward increase in net primary productivity. Environmental conditions, such as specific conductance and water transparency, may have influenced primary productivity along the riverine to freshwater tidal continuum through their effect on both species composition and growth rate. Data suggest light was not the sole controlling factor for primary productivity in this highly turbid estuary; phytoplankton growth rate did not increase when riverine plankton communities from low light conditions upstream were exposed to higher light conditions downstream. This study suggests that the availability of phytoplankton biomass to the estuarine food web may be influenced by management of both phytoplankton growth and community composition along the riverine to freshwater tidal continuum.     

Alternation of factors limiting phytoplankton production in the Cape Fear River Estuary

Phytoplankton nutrient limitation experiments were performed from 1994 to 1996 at three stations in the Cape Fear River Estuary, a riverine system originating in the North Carolina piedmont. Nutrient addition bioassays were conducted by spiking triplicate cubitainers with various nutrient combinations and determining algal response by analyzing chlorophyll a production and ¹⁴C uptake daily for 3 d. Ambient chlorophyll a, nutrient concentration, and associated physical data were collected throughout the estuary as well. At a turbid, nutrient-rich oligohaline station, significant responses to nutrient additions were rare, with light the likely principal factor limiting phytoplankton production. During summer at a mesohaline station, phytoplankton community displayed significant nitrogen (N) limitation, while both phosphorus (P) and N were occasionally limiting in spring with some N+P co-limitation. Light was apparently limiting during fall and winter when the water was turid and nutrient-rich, as well as during other months of heavy rainfall and runoff. A polyhaline station in the lower estuary had clearer water and displayed significant responses to nutrient additions during all enrichment experiments. At this site N limitation occurred in summer and fall, and P limitation (with strong N+P co-limitation) occurred in winter and spring. The data suggest there are two patterns controlling phytoplankton productivity in the Cape Fear system: 1) a longitudinal pattern of decreasing light limitation and increasing nutrient sensitivity along the salinity gradient, and 2) a seasonal alternation of N limitation, light limitation, and P limitation in the middle-to-lower estuary. Statistical analyses indicated upper watershed precipitation events led to increased flow, turbidity, light attenuation, and nutrient loading, and decreased chlorophyll a and nutrient limitation potential in the estuary. Periods of low rainfall and river flow led to reduced estuarine turbidity, higher chlorophyll a, lower ambient nutrients, and more pronounced nutrient limitation.     

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.     

Phytoplankton Functional Groups in a Tropical Estuary: Hydrological Control and Nutrient Limitation

Hydrology and nutrients have been indicated as the main driving factors acting on phytoplankton biomass and composition in estuarine systems, although grazing may occasionally have some influence. In order to identify these factors over temporal and spatial scales, we analyzed physical, chemical, and biological properties of a tropical river-dominated estuary during the dry and rainy seasons. As far as we know, this is the first time that the functional groups approach has been used to analyze the changes in phytoplankton composition in an estuary. This recent framework is based on the tolerances and sensitivities in relation to environmental conditions of groups of species, which are labeled by alpha-numeric codes (Reynolds et al., J. Pl. Res. 24:417–428, 2002). In the estuary of Paraíba do Sul River, all phytoplankton groups were represented by freshwater organisms, indicating the strong influence of the river. However, remarkable shifts in composition and biomass occurred from the low to high flushing seasons, due much more to the river discharge than to nutrient availability. The overall results showed no nitrogen, phosphorus, or silica limitation to phytoplankton growth (mean values: dissolved inorganic nitrogen?=?30.5 µM, soluble reactive phosphorus?=?1.45 µM, and silica?=?208.05 µM). The higher river flow supports a lower phytoplankton biomass composed mainly of nanoplankton (<20 µm) fast-growing functional groups, which are able to maintain biomass even in high flushing conditions (X₁), or large heavy organisms, such as some heavy diatoms of group P, which are able to be in suspension in shallow and turbulent systems. The lower river flow led to the coexistence of large organisms (>20 µm) of the groups P and F, which include slow-growing populations typically found in mesotrophic lakes. Although the functional group approach was originally developed for temperate lakes, our data support this approach for a tropical estuarine environment.     

Nutrient and phytoplankton dynamics in a sewage-impacted Gulf coast estuary: A field test of the PEG-model and equilibrium resource competition theory

Eutrophication and noxious bloom events are becoming more prevalent with increasing anthropogenic activities. To lessen ecological damage, there is a need to develop phytoplankton management programs aimed at enhancing growth of beneficial algae. The success of such management schemes with be dependent on the predictability of phytoplankton succession within the target system to a controlled perturbation. Freshwater lakes appear to exhibit a degree of predictability as described by the PEG-model and Equilibrium Resource Competition theory. We investigated whether these concepts could be applied to a marine system, the Nueces River estuary, Texas. The PEG-model predicted nicely the initial occurrence of edible phytoplankton forms after a favorable nutrient perturbation. Equilibrium Resource Competition theory, however, only successfully predicted the occurrence of major phytoplankton taxa immediately after a nutrient perturbation. Systemwide correlations between N:P and cyanobacteria, green algae, and diatoms were poor. In the Nueces River estuary, succession within the phytoplankton community showed a degree of predictability to nutrient perturbations. Therefore, management of the phytoplankton community composition may be possible. The PEG-model appears to be a useful guide for a phytoplankton management scheme, while the utility of Equilibrium Resource Competition may be limited.     

Land cover and land use change related to shrimp farming in coastal areas of Quang Ninh,Vietnam using remotely sensed data

Rapid development of shrimp farming may lead to unrecognized and undesirable changes of land cover/land use patterns in coastal areas. Of special concern is the loss of mangrove forest in coastal areas such as Quang Ninh, Vietnam, which is adjacent to the World Heritage-listed Ha Long Bay. Understanding the status and changes of land cover/land use for coastal shrimp farms and mangrove forests can support environmental protection and decision-making for sustainable development in coastal areas. Within this context, this paper uses the 1999/2001 Landsat ETM+ and the 2008 ALOS AVNIR-2 imagery to investigate the contraction and expansion of shrimp farms and mangrove forests in coastal areas of Ha Long and Mong Cai, which now have a high concentration of intensive and semi-intensive shrimp farms. Images were separately analyzed and classified before using post-classification comparisons to detect land cover/land use changes in the study area. The results of this study found that the area of mangrove forest has been reduced by an estimated 927.5 ha in Ha Long and 1,144.4 ha in Mong Cai, while shrimp farming areas increased by an estimated 1,195.9 and 1,702.5 ha, respectively, over the same period. The majority of shrimp farms in Mong Cai were established at the expense of mangrove forest (49.4 %) while shrimp farms in Ha Long were mainly constructed on areas previously occupied by bare ground (46.5 %) and a significant proportion also replaced mangroves (23.9 %). The remarkable rate of mangrove loss and shrimp farming expansion detected in this study, over a relatively short time scale indicate that greater awareness of environmental impacts of shrimp farm expansion is required if this industry is to be sustainable, the important estuarine and coastal marine ecosystems are to be protected over the long term, and the capturing and storing of carbon in mangrove systems are to be enhanced for global climate change mitigation and for use as carbon offsets.     

Episodic Biogeochemical Variability in a Low-Flow Mediterranean Estuary

To investigate to what extent episodic physical processes regulate nutrient availability and phytoplankton assemblages of the Mahon estuary (Minorca Island), we carried out an intensive field study during 2010–2011. During the study period, environmental conditions spanned from intense stratification to a continuous mixing and from lack of riverine inflow to intense runoff. Our data reveals a sequence of biogeochemical states of the estuary that result from the interplay between runoff, other non-periodic forcings (winds, sea level oscillations), and variations in water renewal. Seasonal runoff was revealed as a major driver of winter circulation and of the influx of inorganic nutrients, in particular nitrate. However, because of the combination between runoff and flushing time, the effects of floodwater events on phytoplankton are short-lived (days). Conversely, during summer, when freshwater influx declines, water renewal relies on pulsed atmospheric forcing that may be of local or remote origin. As depicted from the low nitrate concentrations (<1 μM) and enhanced ammonium (>1 μM), this change in circulation and external loads carries nutrient assimilation within the estuary head and forces the use of remnant nutrients through regenerating pathways to sustain an enhanced phytoplankton biomass at the lower estuary. Episodic variability represented between 52 and 65% of the annual chlorophyll variance. Despite the fact that episodic pulses represented intense departures from base biogeochemical state of the estuary, at time scale larger than weeks, the phytoplankton community composition and dynamics was largely regulated by the integrated effect of these episodes and other environmental drivers associated with seasonality rather than by individual storm events only. Our results suggest that even though the system presents good recovery capacity to individual storm episodes, it may be more vulnerable to increased nutrient fluxes during summer, as well as to changes in episode timing and frequency.     

Spatial and temporal patterns of factors influencing phytoplankton in a salt wedge estuary, the Swan River, Western Australia

During 1995 the phytoplankton in the Swan River were intensively sampled to assess biomass and species composition. Continuous measurements of fluorescence, salinity, and temperature were made weekly during 40 km sampling trips along the estuary and used to map the seasonal progression of the algal biomass. Weekly measurements of primary production were made and used to model net primary production from the vertical distribution of biomass, irradiance, and phytoplankton species composition. Potential nutrient limitation was assessed with “all but one” nutrient bioassays. The results indicate a complex mixture of potentially limiting factors, which vary in time and space. Although the data sequence is short, it suggests a annual succession pattern of diatoms, chlorophytes, diatoms, and finally dinoflagellates and cryptophytes in late summer-autumn. Peak seasonal biomass was observed during January to April. Mean annual chlorophylla biomass was greatest in upstream stations (5–9), where estimates of net primary production rates averaged 1.55 g C m^?2 d^?1 and gross primary production was 800–1000 g C m^?2 yr^?1. Potential nutrient limitation was most severe from November to May, although not during January 1995. Based on bioassay results, during the period of greatest potential for nutrient limitation, nitrogen was 15 to 30 times more limiting to biomass development than phosphate. Runoff due to consistent rainfall during winter eventually breaks down stratification and flushes the estuary with low-salinity, nutrient-rich water, producing, a light-limited, nutrient-rich aquatic ecosystem. Timing and magnitude of physical forcing events, mainly rainfall, appear critical in determining the susceptibility of this ecosystem to summer and autumn algal blooms.     

Mapping Phytoplankton Community Physiology on a River Impacted Continental Shelf: Testing a Multifaceted Approach

Seasonal phosphorus limitation occurs on the Louisiana continental shelf as a result of high nitrogen loads in the spring and early summer. Prior studies have assessed such nutrient limitation by laborious and time-consuming nutrient analyses, enzyme assays, and nutrient addition bioassays. We undertook surface (0.5–1 m) mapping of fast repetition rate fluorescence (FRRF) parameters to assess nutrient limitation in real time on the Louisiana continental shelf and Mississippi River plume from 29 June to 08 July, 2002 in an effort to further understand phytoplankton productivity in this region, as well as to better inform effective nutrient management strategies. Surface nutrient concentrations (NO₃⁻, NO₂⁻, NH₄⁺, PO₄³⁻), chlorophyll a biomass, alkaline phosphatase (AP) activity, and four FRRF parameters: the maximum quantum yield of photochemistry (F _v /F _m ), the functional absorption cross section for PSII, the time constant for Q _A reoxidation, and the connectivity factor, were measured during continuous underway mapping. Results from traditional methods to assess phytoplankton nutrient stress indicated widespread phosphorus limitation from the Mississippi River plume to the Atchafalaya River, manifested as high inorganic N/P ratios and elevated AP activities associated with phytoplankton biomass. The FRRF data indicated complex patterns of phytoplankton physiology that were likely driven by the rapidly changing conditions in local surface waters and heterogeneous phytoplankton community structure. Correlations of nutrient data and enzyme assays with FRRF parameters were significant but low, potentially due to differences in the manner and time scale with which nutrient limitation affects the different techniques used, indicating that further work is needed to interpret FRRF parameters in large, heterogeneous environments such as estuaries and continental shelves.     

Responses of estuarine phytoplankton communities to nitrogen form and mixing using microcosm bioassays

We examined the effects of different forms of nitrogen and mixed versus static conditions on the structure and function of natural Neuse River estuary phytoplankton communities incubated in 66-liter microcosms in March, May, August, and November 1999. Significant differences were found between effects of mixed versus static treatments in three of four experiments, but no differences were observed between effects of different forms of nitrogen. Mixed incubations resulted in higher contributions of diatoms to total community biomass (measured as chlorophylla) than in static tanks in May. Significantly higher rates of carbon fixation were also observed, likely due to increased suspension of diatoms in surface (illuminated) layers of the tanks. In August, we found significantly higher abundances of cyanobacteria, total community biomass, and rates of carbon fixation in static tanks than in tanks that were mixed. In November, static incubations showed significantly higher abundances of cryptophytes resulting in higher total community biomass and rates of carbon fixation in static tanks than in mixed tanks. Nitrogen additions significantly increased total community biomass relative to controls in May and August, indicating that the communities were nitrogen-limited at these times. We conclude that while nitrogen additions may result in increases in phytoplankton biomass when nitrogen is limiting, phytoplankton community structure in the Neuse River Estuary may be determined more by the hydrodynamics of the system (mixing versus stratification) than by the form of nitrogen available for growth.     

Variations in Concentrations and Fluxes of Dimethylsulfide (DMS) from the Indian Estuaries

In order to examine the variations in concentrations of dimethylsulfide (DMS) and its fluxes to the atmosphere, 25 major and medium estuaries from Indian subcontinent were sampled during wet and dry periods. River discharge brought substantial amount of nutrients and suspended particulate matter (SPM) to the Indian estuaries; however, the concentration of phytoplankton biomass was severely limited by latter due to shallowing of photic depth. Bacillariophyceae was the dominant phytoplankton group in the Indian estuaries followed by green algae, Cyanophyceae, and Dinophyceae. Relatively higher concentrations of DMS were observed in the estuaries located along the east (3.6 ± 5.7 nM) than the west coast of India (0.8 ± 0.3 nM) during wet period whereas no significant differences were observed during dry period. The concentrations of DMS were significantly lower during wet than dry period and it was consistent with the phytoplankton biomass. The slope of the relation between DMS and phytoplankton biomass displayed a significant spatial variation due to contribution of different groups of phytoplankton in the Indian estuaries. The concentrations of DMS in the Indian estuaries were higher than other estuaries in the world except some Chinese estuaries. The annual mean flux (1.95 ± 2.5 μmol m^?2 day^?1) from the Indian estuaries is lower than that of other estuaries in the world, except Pearl River estuary due to inhibition of phytoplankton growth by suspended load and low flushing rates.     

Patterns of Phytoplankton Variability in the Tagus Estuary (Portugal)

Data were collected in the Tagus estuary from 1999–2007 on a monthly basis and combined with published results and for several previous years between 1980 and 1995, so that a comprehensive analysis could be performed over a non-continuous 27-year period. Sampling conditions and methods were similar for all datasets. Extreme wet and dry years were observed. River flow was strongly linked to phytoplankton abundance, with the highest biomass attained in dry years. The observed range of annual median Chl a was 1.8–7.6 µg L^?1 and the overall median was 3.5 µg L^?1. Dissolved inorganic nitrogen (DIN) and silicate showed a clear seasonal pattern, with a maximum in winter?spring, indicating a freshwater origin. Although wastewater treatment started in 1990, no difference was detected from 1980 to the present in terms of DIN and phosphorus. The recorded seasonal pattern for biomass with highest values in late spring–summer period is comparable to other temperate tidally influenced ecosystems. In spite of interannual differences in terms of Chl a concentration or the time of the maximum Chl a occurrence, a repeatable pattern could be identified. The mean growth development time for phytoplankton was 163 days (June 12) ranging 129–206 days (May 9–July 26) during the sampling period. No obvious changes in phytoplankton community structure were observed over time: diatoms were always the dominant group, and cryptophytes were relatively abundant throughout autumn–winter. The dominant species have remained essentially the same since 1969. River inflow, light availability, and temperature were the major factors shaping phytoplankton variability patterns. The strong influence of tidal mixing on the estuarine waters appears to lower the risk of potential eutrophication in the Tagus estuary. The lack of change in nutrients and phytoplankton biomass and composition observed in this study is an important contribution towards the assessment of natural variability versus responses to man-induced inputs in this severely anthropogenically disturbed estuary.