Nitrogen limitation of phytoplankton in a shallow embayment in northern Puget Sound

The effect of nutrient enrichments on natural phytoplankton assemblages was examined in six experiments conducted from June to October 1992. Short-term (4 d to 7 d) nutrient enrichment bioassays were incubated in situ in Padilla Bay, a slough-fed estuary in northern Puget Sound, Washington. Ammonium additions (15 μM) significantly (p<0.001) stimulated phytoplankton biomass accumulation during all six experiments. In two experiments, nitrate additions (15 μM) significantly stimulated accumulation of phytoplankton biomass during October, but not September. Addition of phosphate (1.0 μM) or silicate (15 μM) alone did not stimulate phytoplankton biomass accumulation during any of the experiments. In most experiments, phytoplankton response was greatest in combination treatments of ammonium and phosphate. Dissolved inorganic nutrient concentrations in the containers decreased during all incubations, but showed the greatest reduction in treatments receiving nitrogen. Dissolved inorganic nitrogen (DIN) to phosphate (PO₄ ^3?) ratios were below 16∶1 during all experiments, suggesting the potential for nitrogen limitation. In three experiments, the response of photosynthetic nanoplankton (<20 μm) to ammonium additions was compared to that of the total phytoplankton assemblages. Accumulation of nanoplankton biomass exceeded that of the total phytoplankton during two experiments in August but showed no significant response to ammonium additions in October. Results from the bioassays, the low DIN∶PO₄ ^3? ratios, and the reduction in nutrient concentrations in the containers provide evidence for potential nitrogen limitation of phytoplankton production during summer in Padilla Bay.     

Seasonal variability in response of estuarine phytoplankton communities to stress: Linkages between toxic trace elements and nutrient enrichment

We examined individual and interactive effects of two stressors—nutrients (nitrogen [N] and phosphorus [P]) and trace elements (a mix of arsenic [As], copper [Cu], and cadmium [Cd], and in a second experiment also zinc [Zn] and nickel [Ni])—on phytoplankton of the mesohaline Patuxent River, a tributary of Chesapeake Bay. Experiments were conducted in twenty 1-m³ mesocosms. Four mesocosm runs used two levels of nutrient loadings (0.7–1.0 × ambient N loading and enriched to 1.3–1.6 × ambient N loading) crossed with two levels of trace elements (ambient and enriched approximately 2–5 × higher than ambient concentrations) crossed with five progressive levels of ecosystem complexity. To examine seasonal patterns of responses to stressors, data from these experiments were combined with results of a similar experiment conducted during 1996 (Breitburg et al. 1999a). A second mesocosm experiment examined effects of individual and mixed trace elements, both alone and in combination with nutrients, to further examine which nutrient-trace element interactions were important. Nutrients consistently increased phytoplankton productivity and biomass. Most of the increased biomass was created by large centric diatoms, which increased the mean cell size of the phytoplankton community. Trace element additions decreased phytoplankton productivity and biomass, as well as the contribution of large centric diatoms to phytoplankton biomass. When both trace elements and nutrients were added, trace elements reduced nutrient stimulation. Although the magnitude of the response to nutrient additions tended to be somewhat greater in spring, the seasonal patterns of trace element effects, and nutrient-trace element interactions were far more striking with significant responses restricted to spring mesocosm runs. The second experiment indicated that both As and Cu were more inhibitory to phytoplankton in spring than in summer, but As was more inhibitory in the low nutrient treatments and Cu was more inhibitory in the nutrient enrichment treatments. The potential for strong seasonal patterns and high temporal variability in stressor effects and multiple stressor interactiosn will require close attention in the design and interpretation of management-relevant research and monitoring and may indicate the need for seasonally varying management strategies.     

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     

Water quality and phytoplankton as indicators of hurricane impacts on a large estuarine ecosystem

Three sequential hurricanes in the fall of 1999 provided the impetus for assessing multi-annual effects on water quality and phytoplankton dynamics in southwestern Pamlico Sound, North Carolina. Two and a half years of post-hurricane data were examined for short- and long-term impacts from the storms and >100 year flooding. Salinity decreased dramatically and did not recover until May 2000. Inorganic nitrogen and phosphorus concentrations were briefly elevated during the flooding, but later returned to background levels. Dissolved organic carbon concentrations declined through the whole study period, but did not appear to peak as was observed in the Neuse River estuary, a key tributary of the Sound. Light attenuation was highest in the fall to spring following the storms and was best correlated with chlorophylla concentrations. Phytoplankton biomass (chla) increased and remained elevated until late spring 2000 when concentrations returned to pre-storm levels and then cycled seasonally. Phytoplankton community composition varied throughout the study, reflecting the complex interaction between physiological optimal and combinations of salinity, residence time, nutrient availability, and possibly grazing activity. Floodwater advection or dilution from upstream maxima may have controlled the spatial heterogeneity in total and group-specific biomass. The storms produced areas of shortterm hypoxia, but hypoxic events continued during the following two summers, correlating strongly with water column stratification. Nitrogen loading to the southwestern sound was inferred from network analysis of previous nitrogen cycling studies in the Neuse River estuary. Based on these analyses, nutrient cycling and removal in the sub-estuaries would be decreased under high flow conditions, confirming observations from other estuaries. The inferred nitrogen load from the flood was 2–3 times the normal loading to the Sound; this estimate was supported by the substantial algal bloom. After 8-mos, the salinity and chla data indicated the Sound had returned to pre-hurricane conditions, yet phytoplankton community compositional changes continued through the multi-year study period. This is an example of long-term aspects of estuarine recovery that should be considered in the context of a predicted 10–40 yr period of elevated tropical storm activity in the western Atlantic Basin.     

Phytoplankton Community Indicators of Short- and Long-term Ecological Change in the Anthropogenically and Climatically Impacted Neuse River Estuary, North Carolina, USA

Estuarine and coastal systems represent a challenge when it comes to determining the causes of ecological change because human and natural perturbations often interact. Phytoplankton biomass (chlorophyll a) and group-specific photopigment indicators were examined from 1994 to 2007 to assess community responses to nutrient and climatic perturbations in the Neuse River Estuary, NC. This system experienced nutrient enrichment and hydrologic variability, including droughts, and an increase in hurricanes. Freshwater input strongly interacted with supplies of the limiting nutrient nitrogen (N) and temperature to determine the location, magnitude, and composition of phytoplankton biomass. Multi-annual, seasonal, and episodic hydrologic perturbations, including changes in the frequency and intensity of tropical storms, hurricanes and droughts, caused significant shifts in phytoplankton community structure. Climatic oscillations can at times overwhelm anthropogenic nutrient inputs in terms of controlling algal bloom thresholds, duration, and spatial extent. Eutrophication models should incorporate climatically driven changes to better predict phytoplankton community responses to nutrient inputs and other anthropogenic perturbations.     

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.     

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.     

Denitrification rates measured along a salinity gradient in the eutrophic Neuse River estuary, North Carolina, USA

Denitrification rates along a salinity gradient in the eutrophic Neuse River Estuary, North Carolina, were quantified using membrane inlet mass spectrometry (MIMS) within short-term batch incubations. Denitrification rates within the system were highly variable, ranging from 0 to 275 μmol N m⁻² h⁻¹. Intrasite variability increased with salinity, but no significant differences were observed across the salinity gradient. Denitrification rates were positively correlated with sediment oxygen demand at the upstream sampling site where sediment organic carbon levels were lowest. This relationship was not observed in the more saline sampling sites. Denitrification rates were highest during winter. On an annual basis, denitrification accounted for 26% of the dissolved inorganic nitrogen and 12% of the total nitrogen supplied to the system.     

Long-term Changes and Controlling Factors of Phytoplankton Community in the Gulf of Riga (Baltic Sea)

Trends in phytoplankton monitoring data (1976–2008) from the Gulf of Riga were investigated and linked to environmental factors. Annual means of spring phytoplankton biomass correlated to phosphorus input from land and shifts between diatoms and dinoflagellates were attributed to potential Si limitation and time of sampling relative to the spring phytoplankton succession. The summer phytoplankton biomass, which more than doubled over the study period, was related to the abundance of summer copepods that similarly declined. Cyanobacterial blooms proliferated in summer and the proportion of diatoms similarly declined when the winter–spring inorganic N/P ratio was low. The chlorophyte proportion in summer increased over the study period, and this was linked to increasing temperatures favoring their higher growth rates. The dinoflagellate proportion appeared to decrease with temperatures above a threshold of 15.5°C. Although nutrient inputs and their ratios are important factors for the phytoplankton community, this study suggests that climate change and overfishing could be equally important.     

Formation and seasonal evolution of Atlantic menhaden juvenile nurseries in coastal estuaries

A hypothesis on the formation and seasonal evolution of Atlantic menhaden (Brevoortia tyrannus) juvenile nurseries in coastal estuaries is described. A series of cruises were undertaken to capture postmetamorphic juvenile menhaden and to characterize several biological and physical parameters along estuarine gradients. The two study systems, the Neuse and Pamlico rivers in North Carolina, contain important menhaden nursery grounds. Juvenile menhaden abundance was found to be associated with gradients of phytoplankton biomass as evidenced by chlorophylla levels in the upper water column. Fish abundances were only secondarily associated with salinity gradients as salinity was a factor that moderated primary production in the estuary. The persistence of spatial and temporal trends in the distribution of phytoplankton in the Neuse and Pamlico estuaries was reviewed. The review suggested that postmetamorphic juvenile menhaden modify their distribution patterns to match those created by phytoplankton biomass, which in turn makes them most abundant in the phytoplankton maxima of estuaries. Because the location of these maxima varies with the mixing and nutrient dynamics of different estuaries, so will the location of the nursery.     

Hydrologic Variability and Its Control of Phytoplankton Community Structure and Function in Two Shallow,Coastal, Lagoonal Ecosystems: The Neuse and New River Estuaries,North Carolina,USA

Hydrologic conditions, especially changes in freshwater input, play an important, and at times dominant, role in determining the structure and function of phytoplankton communities and resultant water quality of estuaries. This is particularly true for microtidal, shallow water, lagoonal estuaries, where water flushing and residence times show large variations in response to changes in freshwater inputs. In coastal North Carolina, there has been an increase in frequency and intensity of extreme climatic (hydrologic) events over the past 15 years, including eight hurricanes, six tropical storms, and several record droughts; these events are forecast to continue in the foreseeable future. Each of the past storms exhibited unique hydrologic and nutrient loading scenarios for two representative and proximate coastal plain lagoonal estuaries, the Neuse and New River estuaries. In this synthesis, we used a 13-year (1998–2011) data set from the Neuse River Estuary, and more recent 4-year (2007–2011) data set from the nearby New River Estuary to examine the effects of these hydrologic events on phytoplankton community biomass and composition. We focused on the ability of specific taxonomic groups to optimize growth under hydrologically variable conditions, including seasonal wet/dry periods, episodic storms, and droughts. Changes in phytoplankton community composition and biomass were strongly modulated by the amounts, duration, and seasonality of freshwater discharge. In both estuaries, phytoplankton total and specific taxonomic group biomass exhibited a distinctive unimodal response to varying flushing rates resulting from both event-scale (i.e., major storms, hurricanes) and more chronic seasonal changes in freshwater input. However, unlike the net negative growth seen at long flushing times for nano-/microphytoplankton, the pigments specific to picophytoplankton (zeaxanthin) still showed positive net growth due to their competitive advantage under nutrient-limited conditions. Along with considerations of seasonality (temperature regimes), these relationships can be used to predict relative changes in phytoplankton community composition in response to hydrologic events and changes therein. Freshwater inputs and droughts, while not manageable in the short term, must be incorporated in water quality management strategies for these and other estuarine and coastal ecosystems faced with increasing frequencies and intensities of tropical cyclones, flooding, and droughts.     

Composition and vertical flux of organic matter in a large Alaskan estuary

Observations of the composition and rate of input of organic matter to the sea floor were made at three locations in lower Cook Inlet, Alaska, during five cruises taken in the spring and summer of 1978. Total particulate, plant pigment, carbon, nitrogen, fecal pellet, and phytoplankton cell fluxes, inferred from sediment trap samples, were related to algal biomass and production in overlying waters. A daily average of 7.5% of the phytoplankton biomass was lost to the bottom. Of this loss, 83% was attributable to zooplankton grazing and fecal pellet production. At the three sampling sites, an average of 39 g C m^?2 (range of 17–60 g C m^?2, was sedimented to the bottom between May and August. This carbon flux represented an average of 12% of the total primary production measured for that time period. Kachemak Bay eastern arm of the inlet, is identified as an extremely productive embayment in which large amounts of organic matter were transferred to the sea floor.     

Seasonal coupling between ciliate and phytoplankton standing stocks in the South Slough of Coos Bay,Oregon

The standing stocks of ciliates and phytoplankton together with physical variables (temperature and density) were measured biweekly from March 6, 1999, to March 22, 2000, in the marine-dominated region of the South Slough, the southern arm of the Coos Bay estuary (Oregon, United States). The abundance and biomass of naked ciliates correlated significantly with phytoplankton <5 μm (ultraphytoplankton) biomass throughout the sampling periods and with total phytoplankton biomass between October and march; possibly due to a compositional shift in the >5 μm phytoplankton fraction from diatoms in the spring-summer period to flagellates during this fall-winter period. Temperature could explain 49% of the variation of ultraphytoplankton and naked ciliate biomass across seasons and may be important in determining the rate of the ciliate numerical response to increases in ultraphytoplankton and its assimilation into ciliate biomass. High standing stocks of ciliates, their strong coupling with ultraphytoplankton across seasons, and the relatively higher contribution of ciliate carbon to the ciliate and >5 μm phytoplankton carbon pool in the October–March period suggest that ciliates are a significant component to the South Slough food web and may be particularly important during seasons of reduced phytoplankton biomass.     

Using stable isotope analyses to assess carbon dynamics in a shallow subtropical estuary

The sources of carbon, which fuel water column respiration, remain unresolved for most estuaries; our objective was to examine carbon dynamics in a shallow subtropical estuary. We sampled the Sabine-Neches estuary, Texas, during low (November 1999) and high (May 2000) freshwater inflow and measured stable carbon isotope ratios of the dissolved inorganic and orgnaic carbon (δ¹³C-DIC, δ¹³C-DOC), as well as quantifying accessory parameters (salinity, nutrients, total suspended solids, and photosynthetic pigments). Pigment analysis indicated that diatoms were the predominant phytoplankton. Data from the May 2000 sampling event exhibited conservative mixing, indicating that the system was acting as a conduit between the watershed and the Gulf of Mexico. During November, mixing was generally nonconservative indicating extensive recycling of allochthonous and autochthonous carbon sources. Our data imply that both carbon sources had similar isotope, ratios that made it impossible to unambiguously determine the dominant source supporting respiration. The nonconservative DIC concentration data indicating an autotrophic sink as well as the strong relationship between δ¹³C-DOC and chlorophylla, suggest that in situ production was an important component of the DOC pool. We hypothesize that uncharacteristically calm wind conditions during sampling may have promoted phytoplankton settling, removing autotrophs, from the water column, but leaving behind a dissolved biogeochemical signature. Interpretation of carbon dynamics may be confounded by spatial and temporal decoupling of producers and consumers from biogeochemical indicators.     

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.     

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.     

The Influence of Coastal Nutrients on Phytoplankton Productivity in a Shallow Low Inflow Estuary,Drakes Estero,California (USA)

Seasonal wind-driven upwelling along the U.S. West Coast supplies large concentrations of nitrogen to surface waters that drives high primary production. However, the influence of coastal upwelled nutrients on phytoplankton productivity in adjacent small estuaries and bays is poorly understood. This study was conducted in Drakes Estero, California, a low inflow estuary located in the Point Reyes National Seashore and the site of an oyster mariculture facility that produces 40 % of the oysters harvested in California. Measurements of nutrients, chlorophyll a, phytoplankton functional groups, and phytoplankton carbon and nitrogen uptake were made between May 2010 and June 2011. A sea-to-land gradient in nutrient concentrations was observed with elevated nitrate at the coast and higher ammonium at the landward region. Larger phytoplankton cells (>5 μm diameter) were dominant within the outer and middle Estero where phytoplankton primary productivity was fueled by nitrate and f-ratios were >0.5; the greatest primary production rates were in the middle Estero. Primary production was lowest within the inner Estero, where smaller phytoplankton cells (<5 μm) were dominant, and nitrogen uptake was dominated by ammonium. Phytoplankton blooms occurred at the outer and middle Estero and were dominated by diatoms during the spring and dry-upwelling seasons but dinoflagellates during the fall. Small flagellated algae (>2 μm) were dominant at the inner Estero where no blooms occurred. These results indicate that coastal nitrate and phytoplankton are imported into Drakes Estero and lead to periods of high new production that can support the oyster mariculture; a likely scenario also for other small estuaries and bays.     

Seasonal changes in the abundance and composition of plant pigments in particulate organic carbon in the lower Mississippi and Pearl Rivers

Plant pigments in particulate organic carbon were examined in the lower Mississippi and Pearl Rivers (U.S.), along with physical variables and nutrients to study seasonal changes in the abundance and composition of phytoplankton. Water samples were collected monthly from September 2001 to August 2003 in the lower Mississippi River (MR; no samples were taken in February 2002) and from August 2001 to July 2003 in the Pearl River (PR). High concentrations of total suspended solids (TSS), nutrients, and chlorophylla (chla; dominated by diatoms) were observed in the lower MR. The smaller blackwater PR was characterized by lower nutrients and chla, higher ultraviolet absorbance, and a phytoplankton biomass dominated by chlorophytes. Chla concentrations in the lower MR was high in summer low-flow periods and also during interims of winter and spring, and did not couple with physical variables and nutrients, likely due to a combination of in situ production and inputs from reservoirs, navigation locks and oxbow lakes in the upper MR and Missouri River. Chla concentrations in the PR was only high in summer low-flow periods and were controlled by temperature and concentrations of chromophoric dissolved organic matter 9CDOM). The high, diatom-dominated phytoplankton biomass in the lower MR was likely the result of decreasing TSS (increased damming in the watershed) and increasing nutrients (enhanced agricultural runoff) over the past few decades. Lower phytoplankton biomass (dominated by chlorophytes) in the PR was likely linked with intense shading by CDOM and lower availability of nutrient inputs. An increase in the relative importance of phytoplankton biomass in large turbid rivers, such as the MR, could have significant effects on the age and lability of riverine organic matter entering the ocean, the stoichiometric balance of nutrients delivered to coastal margins, and the sequestration of atmospheric CO₂ in these dynamic regions.     

Sublethal Effects of Crude Oil on the Community Structure of Estuarine Phytoplankton

While the ecological impacts of crude oil exposure have been widely studied, its sublethal effects on phytoplankton community structure in salt marsh estuaries have not been well documented. The purpose of this study was to simulate oil spill conditions using a microcosm design to examine short-term (2 day) changes in phytoplankton community composition and total biomass following exposure to crude oil obtained from the Deepwater Horizon oil spill and a mixture of Texas crude oils. Microcosm experiments were performed in situ in North Inlet Estuary near Georgetown, SC. A control and six replicated experimental treatments of crude oil additions at final concentrations of 10, 50, or 100 μl l⁻¹ of either Deepwater Horizon spill oil or the Texas crude mixture were incubated under in situ conditions. Photopigments were analyzed using high-performance liquid chromatography and community composition was determined using ChemTax. Total phytoplankton biomass (as chl a) declined with increasing crude oil concentrations. Prasinophytes, the most abundant microalga in both experiments, showed no response to oil exposure in one experiment and a significant negative response in the other. Diatoms euglenophytes and chlorophytes appeared relatively resistant to oil contamination at the exposure levels used in this study, maintaining or increasing their relative abundance with increasing oil concentrations. Chlorophytes and cyanobacteria increased in relative abundance while cryptophyte abundance decreased with increasing oil concentrations. The results of these experiments suggest that low levels of crude oil exposure may reduce total biomass and alter phytoplankton community composition with possible cascade effects at higher trophic levels in salt marsh estuaries.     

Spectral Irradiance, Phytoplankton Community Composition and Primary Productivity in a Salt Marsh Estuary, North Inlet, South Carolina, USA

We investigated spatial and temporal changes in spectral irradiance, phytoplankton community composition, and primary productivity in North Inlet Estuary, South Carolina, USA. High concentrations of colored dissolved organic matter (CDOM) were responsible for up to 84 % of the attenuation of photosynthetically available radiation (PAR). Green-yellow wavelengths were the predominant colors of light available at the two sampling sites: Clam Bank Creek and Oyster Landing. Vertical attenuation coefficients of PAR were 0.7–2.1 m^?1 with corresponding euphotic zone depths of 1.5–6.7 m. Phytoplankton biomass (as chlorophyll a [chl a]) varied seasonally with a summer maximum of 16 μg chl a l^?1 and a winter minimum of 1.4 μg chl a l^?1. The phytoplankton community consisted mainly of diatoms, prasinophytes, cryptophytes and haptophytes, with diatoms and prasinophytes accounting for up to 67 % of total chl a. Changes in phytoplankton community composition showed strongest correlations with temperature. Light-saturated chl a-specific rates of photosynthesis and daily primary productivity varied with season and ranged from 1.6 to 14 mg C (mg chl a) ^?1?h^?1 (32–803 mg C m^?3?day^?1). Calculated daily rates added up to an annual carbon fixation rate of 84 g C m^?3?year^?1. Overall, changes in phytoplankton community composition and primary productivity in North Inlet showed a strong dependence on temperature, with PAR and spectral irradiance playing a relatively minor role due to short residence times, strong tidal forcing and vertical mixing.