Cryptic Blooms: Are Thin Layers the Missing Connection?

Harmful algal blooms (HABs) are common in Monterey Bay, CA, and have resulted in repeated closures of shellfish fisheries and the poisoning and death of marine mammals. In the majority of instances, HAB events in this region are first detected by the presence of sick or dying animals. The phrase “cryptic blooms” was adopted to denote the appearance of poisoning at higher trophic levels with no prior evidence of a large phytoplankton bloom. We hypothesize that the onset of many HAB events goes undetected because the bloom is initially concentrated in discrete thin subsurface layers in the water column that are easily missed by conventional sampling and monitoring methods. In this paper, we report on the detection and monitoring of a subsurface layer of phytoplankton in northern Monterey Bay, CA, using a high-resolution, autonomous profiler. This ‘thin layer,’ which measured from 10 cm to 3 m in thickness (85% < 2 m; 54% < 1 m), persisted over a 7-day period near the base of the pycnocline. The phytoplankton assemblage in the layer was primarily composed of a multi-species assemblage of Pseudo-nitzschia including the toxin-producing species Pseudo-nitzschia australis. Concentrations of toxic phytoplankton (P. australis), cyanobacteria, and bacteria in the layer were significantly higher than outside the layer (P < 0.05). Counts of total Pseudo-nitzschia spp. showed similar levels of enrichment in the layer compared to outside the layer. Our findings indicate that, when monitoring for HABs, it is critical to sample at scales appropriate to resolve thin layers. Thin layers have been identified as a common recurrent feature in a variety of coastal systems, suggesting that the use of autonomous high-resolution vertical profilers coupled with targeted sampling, could allow more timely detection of HABs in many coastal environments.     

Changes in Phytoplankton Biomass in the Western Scheldt Estuary During the Period 1978–2006

We investigated whether climate change results in long-term changes in phytoplankton biomass and phenology in a turbid eutrophic coastal plain estuary. Changes in annual mean chlorophyll a (chla) concentrations were studied for the period 1978–2006 in the eutrophic and turbid macro-tidal Western Scheldt estuary. Three stations were investigated: WS1, at the mouth of the estuary; station WS6, halfway up the estuary; and station WS11, near the Dutch–Belgian border near the upstream end of the estuary. No significant long-term changes in yearly averaged chla concentrations were observed in WS1 and WS6, but in WS11 the phytoplankton biomass decreased considerably. This is most likely due to an increase in grazing pressure as a result of an improvement in the dissolved oxygen concentrations. Spectral analyses revealed a possible periodicity of 7 years in the mean chla which was related to periodicity in river discharge. We also observed strong phenological responses in the timing of the spring/summer bloom which were related to a well-documented increase in the temperature in the estuary. The fulcrum, the center of gravity or the day at which 50% of the cumulative chla was reached during the year, advanced by 1–2 days/year. A similar trend was observed for the month in which the maximum bloom was observed, with the exception of station WS1. All stations showed an earlier initiation of the bloom, whereas the day at which the phytoplankton bloom was terminated also moved forward in time excepted for WS11. As a result, the bloom length decreased at station WS1, remained the same at station WS6, and increased at WS11. This complicated pattern in bloom phenology demonstrates the complex nature of ecosystem functioning in estuaries.     

Long-term Dynamics of Phytoplankton in the Rhode River,Maryland (USA)

We analyzed interannual variability in a long-term record of chlorophyll concentration and phytoplankton species composition in the Rhode River, Maryland (USA). Over the approximately 30-year record, there was no long-term monotonic trend in phytoplankton chlorophyll concentration, though temporary directional trends related to precipitation patterns sometimes persisted for a decade before reversing. From counts on preserved samples, we estimated the dominance by different pigment-bearing groups and size classes. Diatoms, dinoflagellates, and cryptophytes comprised about 80–97% of the annual averaged class-specific biovolume. Cryptophytes dominated the first 2 years in the data set but displayed a long-term decline, after which diatoms dominated in all but four consecutive years in which there were large dinoflagellate blooms. There was a long-term increase in cells with equivalent spherical diameters from 4 to 10 μm, accompanied by declines in the proportion of cells in the 2- to 4- and 10- to 20-μm size classes. The main cause of these changes in size classes was a long-term increase in Chroomonas sp. and Apedinella radians and long-term decline in Microcystis sp. and an unidentified cryptophyte, respectively. These taxa were cosmopolitan in their seasonal and spatial distributions, and hence the long-term changes in taxa did not conform well with conceptual models based on succession of “life-forms.” The segregation of diatoms and dinoflagellates expected on the basis of “life-forms” applied to seasonal and spatial patterns. Characterizing the phytoplankton community in terms of diversity, size, and class-specific biovolume gave results that were consistent with one another and added insight to the broad-scale changes in chlorophyll concentration. Subtle changes in the size distribution of cryptophytes were not apparent from biovolume measures and would not have been apparent without microscopy. Though causes of such shifts may be difficult to identify, identifying the causes and predicting potential consequences cannot even be attempted without awareness of the phenomena.     

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.     

Fine-scale responses of phytoplankton to freshwater influx in a tropical monsoonal estuary following the onset of southwest monsoon

In May of 2007, a study was initiated by the National Institute of Oceanography (NIO), Goa, India, to investigate the influence of monsoonal rainfall on hydrographic conditions in the Mandovi River of India. The study was undertaken at a location ∼2 km upstream of the mouth of this estuary. During the premonsoon (PreM) in May, when circulation in the estuary was dominated by tidal activity, phytoplankton communities in the high saline (35–37 psu) waters at the study site were largely made up of the coastal neritic species Fragilaria oceanica, Ditylum brightwellii and Trichodesmium erythraeum. During the later part of the intermonsoon (InterM) phase, an abrupt decline in salinity led to a surge in phytoplankton biomass (Chlorophyll a ∼14 mg m^− 3), of a population that was dominated by Thalassiosira eccentricus. As the southwest monsoon (SWM) progressed and the estuary freshened salinity and Chlorophyll a (Chl a) concentrations decreased during the MoN, Skeletonema costatum established itself as the dominant form. Despite the low biomass (Chl a <2 mg m^− 3), the phytoplankton community of the MoN was the most diverse of the entire study. During the postmonsoon (PostM), the increase in salinity was marked by a surge in dinoflagellate populations comprising of Ceratium furca, Akashiwo sanguinea, and Pyrophacus horologium.     

Dinoflagellate Cysts in Coastal Sediments as Indicators of Eutrophication: A Case of Gwangyang Bay,South Sea of Korea

Diatom densities in the surface water and dinoflagellate cysts in bottom sediments of Gwangyang Bay were studied to determine changes in the phytoplankton community structure in response to anthropogenic eutrophication and to assess the use of dinoflagellate cysts as indicators of coastal eutrophication. Our results show that, in nutrient-enriched environments, diatoms are particularly benefited from the nutrients supplied and, as a consequence, heterotrophic dinoflagellates that feed on the diatoms can be more abundant than autotrophic dinoflagellates. In short-core sediment records, a marked shift in autotrophic–heterotrophic dinoflagellate cyst compositions occurred at a depth of approximately 9–10 cm corresponding to the timing of the 1970s industrialization around Gwangyang Bay. This tentatively indicates that diatom and dinoflagellate communities here have undergone a considerable change mainly due to increased nutrient loadings from both domestic sewage effluent and industrial pollution. Our study suggests a possible potential use of dinoflagellate cysts in providing retrospective information on the long-term effects of coastal eutrophication.     

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.     

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.     

Erratum to: Comparative Analysis of Phytoplankton Composition and Abundance over a Two-Decade Period at the Land<Emphasis Type="Italic">–</Emphasis>Ocean Boundary of a Tropical Mangrove Ecosystem

Inter-annual variations of phytoplankton abundance and community organization were observed over a two-decade period along with the ancillary parameters at the land–ocean boundary associated with the Sundarban mangrove forest (21°32′ and 22°40′ N and 88°05′ and 89° E), along the NE Coast of the Bay of Bengal. The number of definable Bacillariophyceae species exceeded Dinophyceae taxa, and the total number of bloom-forming species declined from a maximum of ten in 2000 and a minimum of two in 2007. Blooms of the diatom Coscinodiscus radiatus were common in 2000 and 2007. Tide cycles and the onset of the monsoon season played important roles in diurnal and seasonal variability of phytoplankton. Phytoplankton biovolume showed seasonality, with the highest levels during post-monsoon periods and lowest levels during the monsoon period. Phytoplankton abundance was correlated to rainfall patterns, which may be altered by long-term changes in climate.     

Phytoplankton, nutrients, and transparency in Danish coastal waters

We present a comparative analysis of 1400 data series of water chemistry (particularly nitrogen and phosphorus concentrations), phytoplankton biomass as chlorophylla (chla) concentrations, concentrations of suspended matter and Secchi depth transparency collected from the mid-1980s to the mid-1990s from 162 stations in 27 Danish fjords and coastal waters. The results demonstrate that Danish coastal waters were heavily eutrophied and had high particle concentrations and turbid waters. Median values were 5.1 μg chla 1⁻¹, 10.0 mg DW 1⁻¹ of suspended particles, and Secchi depth of 3.6 m. Chlorophyll concentration was strongly linked to the total-nitrogen concentration. The strength of this relationship increased from spring to summer as the concentration of total nitrogen declined. During summer, total nitrogen concentrations accounted for about 60% of the variability in chlorophyll concentrations among the different coastal systems. The relationship between chlorophyll and total phosphorus was more consistant over the year and correlations were much weaker than encountered for total nitrogen. Secchi depth could be predicted with good precision from measurements of chlorophyll and suspended matter. In a multiple stepwise regression model with In-transformed values the two variables accounted for most of the variability in water transparency for the different seasons and the period March–October as a whole (c. 80%). We were able to demonstrate a significant relationship between total nitrogen and Secchi depth, with important implications for management purposes.     

Influence of hydrology on phytoplankton species composition and life strategies in a subtropical coastal lagoon periodically connected with the Atlantic Ocean

A survey was carried out to investigate the relationship of phytoplankton biovolume, structure, and species life strategies with major abiotic factors in a subtropical choked coastal lagoon (34°33′S, 54°22′W) naturally connecting with the Atlantic Ocean several times a year. Marine and limnetic influence areas were sampled on a monthly basis during two periods, one of low rainfall and high conductivity (August 1996 to February 1998) and a second period with the opposite tendency (December 1998 to March 2000). Photosynthetically active radiation availability was high and reached the bottom (>1% of the incident light), while dissolved inorganic nitrogen (0.6–18.4 μM), soluble reactive phosphorus (<0.3–2.7 μM), and reactive silica (5–386 μM) were highly variable. Life strategies were identified in the phytoplankton as a function of morphology. C-strategists, invasive planktonic and epipelic species of small size, and R-strategists, mixing-dependent species of medium size, characterized this permanently mixed system. High frequency of exchange with the ocean prevented high biomass accumulation. Phytoplankton biomass was lower in the second period of high rainfall (2.3 and 1.1 mm³ 1⁻¹ for period 1 and 2 respectively). A canonical correspondence analysis showed that conductivity, nitrogen, phosphorus, and silica were the main environmental variables explaining phytoplankton species composition patterns. During the first period, Bacillariophyceae (mostly pennate species) and the potentially toxicPrococentrum minimum were dominant; during the second period a higher contribution of flagellates (Cryptophyceae, Euglenophyceae, Prasinophyceae, and flagellates <7 μm) was found. Differences of phytoplankton biomass, main taxonomic groups, and strategies were found between periods but not between limnic and marine areas, suggesting that hydrological dynamic is more relevant than seasonal and spatial differences.     

Phytoplankton Interannual Variability at Cassino Beach, Southern Brazil (1992–2007), with Emphasis on the Surf Zone Diatom Asterionellopsis glacialis

A long-term study (monthly sampling, 1992 to 2007) was conducted in the surf zone of Cassino Beach, Southern Brazil, in order to detect possible natural and/or anthropogenic disturbances. Surface water temperature (6–29°C) was the only parameter with predictable seasonal variation; salinity (14–38) was inversely related to rainfall (3.1–485.2 mm month⁻¹) and low values followed extreme precipitation periods in 1997/1998 and 2002/2003 (El Ni?o years). Asterionellopsis glacialis and chlorophyll a presented high concentrations and peak frequency until 1998, when an intense mud deposition occurred with concomitant extreme rainfall. It affected the surf zone and beach, changing the hydrology and dissolved inorganic nutrient availability. Six phytoplankton species groups were recognized with distinct responses to this mud deposition. We conclude that large-scale climatic changes, like El Ni?o Southern Oscillation, in conjunction with human activities significantly altered the phytoplankton ecology of the highly dynamic Cassino Beach surf zone.     

Cell-Specific Alkaline Phosphatase Expression by Phytoplankton from Winyah Bay,South Carolina,USA

Alkaline phosphatase expression by phytoplankton from two sites in Winyah Bay, SC, USA was investigated using nutrient-addition bioassays and cell-specific enzyme-labeled fluorescence (ELF) measurements. Our aim was to determine whether expression was group- or species-specific within the phytoplankton community. Diatoms dominated the riverine site in May, the coastal site in July, and both sites in August. Phytoplankton growth was limited by nitrogen (N) availability at the coastal site in May and the riverine site in August, but phosphate limitation was not observed. Alkaline phosphatase expression ranged from ∼30% of cells enumerated to less than 1% and was significantly reduced by inorganic phosphorus (P; 10 μM P) additions. Expression was restricted to species with low abundance, and there were no shifts in community composition consistent with alkaline phosphatase expression. Lack of phosphate limitation at higher-than-Redfield N/P ratios (up to 40:1), however, points to a potentially wider role of dissolved organic phosphorus in nutrition of Winyah Bay phytoplankton than indicated by the ELF assay.     

Chronic food limitation of egg production in populations of copepods of the genusAcartia in the San Francisco estuary

Egg production of planktonic copepods, is commonly measured as a proxy for secondary production in population dynamics studies and for quantifying food limitation. Although limitation of copepod egg production by food quantity or quality is common in natural waters, it appears less common or severe in estuaries where food concentrations are often high. San Francisco Estuary, California, has unusually low concentrations of chlorophyll compared to other estuaries. We measured egg production rates of three species ofAcartia, with dominate the zooplankton biomass at salinity above 15 psu, on 36 occasions during 1999–2002. Egg production was determined by incubating up to 40 freshly collected individual copepods for 24 h in 140 ml of ambient water. Egg production was less than 10 eggs female⁻¹ d⁻¹ most of the year, but as high as 52 eggs female⁻¹ d⁻¹ during month-long spring phytoplankton blooms. Egg production was a saturating function of total chlorophyll concentration with a mean of 30 eggs female⁻¹ d⁻¹ above a chlorophyll concentration of 12±6 mg chl m⁻³. We take chlorophyll to be a proxy for total food ofAcartia, known to feed on microzooplankton as well as phytoplankton. These findings, together with long-term records of chlorophyll, concentration and earlier studies of abundance of nauplius larvae in the estuary, imply chronic food limitation ofAcartia species, with sufficient food for maximum egg production <10% of the time over the last 25 yr. These results may show the most extreme example of food limitation of copepod reproduction in any temperate estuary. They further support the idea that estuaries may provide suitable habitat forAcartia species by virtue of other factors than high food concentration.     

Variations in stable carbon isotope ratio of the copepod Acartia tonsa during the onset of the Texas brown tide

The Laguna Madre of South Texas is a shallow coastal lagoon whose dominant primary producers shifted from seagrasses to phytoplankton with the onset of the Texas brown tide, which persisted from 1990 through 1997. Acartia tonsa is the dominant component of the mesozooplankton and forms an important link in both the phytoplankton and detritus-based pelagic food webs. Stable carbon isotope ratios of A. tonsa, as well as the two major primary producers: phytoplankton (as particulate organic carbon) and seagrasses, were measured from March 1989 to October 1991. Zooplankton samples were collected at four locations in the Laguna Madre: two in shallow water (c. 1 m) over seagrass beds and two in slightly deeper water (c. 2–3 m) over a muddy bottom in a secondary bay without seagrasses. We found seasonal trends in the isotopic composition of A. tonsa collected within both habitats as well as distinct differences between the average {ie995-1} values of individuals collected in the two regions. Isotopic ratios of animals collected during the summer months were generally 4–8‰ enriched in ¹³C compared with those collected in the winter, at all stations. A. tonsa collected over seagrass beds were 2–5‰ more enriched in ¹³C than those collected over muddy bottoms. These observations suggest carbon derived from seagrasses can be an important source of nutrition for these copepods in summer, especially for copepods living over seagrass beds. The effects of the persistent brown tide decreased the contribution of seagrasses as a carbon source for A. tonsa during the summer of 1991. The pathway by which seagrass carbon enters the diet of A. tonsa is unclear, but the two pathways considered most likely are through copepods feeding on microzooplankton that have fed on bacteria nourished on seagrass carbon, or by copepods feeding directly on particles of seagrass detritus.     

Microzooplankton grazing and nitrogen excretion across a surface estuarine-coastal interface

The role of the microzooplankton community in regulating phytoplankton biomass was examined across a gradient from a river-dominated estuary to an oceanic-influenced coastal zone. Three stations located along a salinity gradient from the central region of Mobile Bay to 10 km off the coast were sampled from May 1994 to August 1995. Microzooplankton herbivory rates on phytoplankton and microzooplankton excretion of nitrogen derived from phytoplankton were estimated using the dilution technique. Microzooplankton grazing rates (range of station means=0.57–1.10 d⁻¹) and phytoplankton growth rates (0.70–1.62 d⁻¹) both increased across the salinity gradient from the bay station to the offshore station. However, the percent of primary production grazed per day was highest at the bay station (mean=83%) and decreased to a low at the offshore station (mean=64%). Excretion of phytoplankton-derived nitrogen by the microzooplankton was greatest at the bay and bay mouth stations. Excreted nitrogen could potentially supply 39%, 29%, and 20% of phytoplankton nitrogen demand at the bay, bay mouth, and offshore stations, respectively. These results support the idea that herbivorous microzooplankton are important in mediating nitrogen flow to both lower and higher trophic levels. *** DIRECT SUPPORT *** A01BY085 00012     

Meso-scale atmospheric events promote phytoplankton blooms in the coastal Bay of Bengal

The Bay of Bengal is considered to be a low productive region compared to the Arabian Sea based on conventional seasonal observations. Such seasonal observations are not representative of a calendar year since the conventional approach might miss episodic high productive events associated with extreme atmospheric processes. We examined here the influence of extreme atmospheric events, such as heavy rainfall and cyclone Sidr, on phytoplankton biomass in the western Bay of Bengal using both in situ time-series observations and satellite derived Chlorophyll a (Chl a) and sea surface temperature (SST). Supply of nutrients through the runoff driven by episodic heavy rainfall (234 mm) on 4–5 October 2007 caused an increase in Chl a concentration by four times than the previous in the coastal Bay was observed within two weeks. Similar increase in Chl a, by 3 to 10 times, was observed on the right side of the cyclone Sidr track in the central Bay of Bengal after the cyclone Sidr. These two episodic events caused phytoplankton blooms in the western Bay of Bengal which enhanced ~40% of fishery production during October–December 2007 compared to that in the same period in 2006.     

Phytoplankton Size and Taxonomic Composition in a Temperate Estuary Influenced by Monsoon

Temporal and spatial variations in phytoplankton in Asan Bay, a temperate estuary under the influence of monsoon, were investigated over an annual cycle (2004). Phytoplankton blooms started in February (>20 μg chl l⁻¹) and continued until April (>13 μg chl l⁻¹) during the dry season, especially in upstream regions. The percentage contribution of large phytoplankton (micro-sized) was high (78–95%) during the blooms, and diatoms such as Skeletonema costatum and Thalassiosira spp. were dominant. The precipitation and freshwater discharge from embankments peaked and supplied nutrients into the bay during the monsoon event, especially in July. Species that favor freshwater, such as Oscillatoria spp. (cyanobacteria), dominated during the monsoon period. The phytoplankton biomass was minimal in this season despite nutrient concentrations that were relatively sufficient (enriched), and this pattern differed from that in tropical estuaries affected by monsoon and in temperate estuaries where phytoplankton respond to nutrient inputs during wet seasons. The flushing time estimated from the salinity was shorter than the doubling time in Asan Bay, which suggests that exports of phytoplankton maximized by high discharge directly from embankments differentiate this bay from other estuaries in temperate and tropical regions. This implies that the change in physical properties, especially in the freshwater discharge rates, has mainly been a regulator of phytoplankton dynamics since the construction of embankments in Asan Bay.     

Assessing the Role of pH in Determining Water Column Nitrification Rates in a Coastal System

Ocean acidification is predicted to impact the nitrogen cycle in a variety of ways. Specifically, manipulations of water column pH have shown that nitrification, the microbial conversion of ammonium to nitrate, is inhibited at low pH. A decrease in nitrification may impact phytoplankton composition and production, denitrification, and the production of nitrous oxide. We compiled an existing unique data set of concurrent water column nitrification rates and water column pH values from a temperate New England estuary (Narragansett Bay, RI, USA). Contrary to the current hypothesis, we found that nitrification rates were highest at low pH and significantly (P = 0.0031) lower at high water column pH. In this study, pH varied up to 0.85 units, 20% more than the maximum predicted ocean pH decrease of 0.7 units. These results highlight that nitrifying organisms in coastal systems tolerate a wide range of pH values. Moreover, the degree of negative correlation with pH may depend on site-specific environmental conditions. Combined, these findings indicate that the current hypothesis of the negative impacts of ocean acidification on nitrification, at least for the coastal ocean, might need reevaluation.     

Interactive effects of hydrology and salinity on oligohaline plant species productivity: Implications of relative sea-level rise

Sea-level rise is anticipated to alter hydrologic and salinity regimes of coastal wetlands. We conducted a mesocosm experiment to determine species-level responses to 12 sea-level rise scenarios. Both hydrologic regime (−10, +5, and +20 cm flooding depth) and salinity level (fresh, 2‰, 4‰ and 6‰) were interactively manipulated. Within these various sea-level rise scenarios, we sought to determine the effects of hydrologic regime, salinity level, and the interaction of these two stresses on the productivity ofPanicum hemitomon, Sagittaria lancifolia, andSpartina patens, which are dominant macrophytes of fresh, intermediate, and brackish marsh types, respectively, in coastal Louisiana and the southeastern coastal plain. We found that altered hydrologic regimes and increased salinity levels differentially affected edaphic conditions and species-level productivity. Increases in flooding depth were most detrimental toS. patens. Salinity levels greater than 4‰ resulted in mortality ofP. hemitomon, and salinity levels of 6‰ resulted in reduced growth and eventual death, ofS. lancifolia. The effects of elevated salinity levels onP. hemitomon andS. lancifolia were exacerbated when coupled with increased flooding levels. Although soil organic matter was shown to increase in all vegetative conditions, increases were dependent upon the productivity of the species under the different hydrologic regimes and salinity levels withP. hemitomon displaying tremendous potential to increase soil organic matter under fresh conditions, especially when coupled with moderate flooding. The results of this study indicate that as plant communities are subjected to long-term changes in hydrology and salinity levels, community productivity and sustainability ulimately will be determined by species-level tolerances in conjunction with species interactions.