Eutrophication is increasingly becoming a problem for freshwater lakes. We evaluated the effects of additions nitrate (N as NO3−) and phosphate (P as PO43−) on phytoplankton in a temperate lake reservoir (Lake Murray, South Carolina). High-performance liquid chromatography and ChemTax were used to measure concentrations of microalgal groups in the lake in 2021–2023 and bioassays. The phytoplankton community during the summer months consisted of green algae (37%), diatoms (27%), cryptophytes (20%), cyanobacteria (11%) and dinoflagellates (4%). Bioassays of N (20-μM NaNO3), P (10-μM KH2PO4) and N + P additions were conducted monthly from April to October 2023. All microalgal groups, except cyanobacteria, exhibited nutrient co-limitation with N as the primary limiting nutrient. Similarly, cyanobacteria exhibited co-limitation, but with P as the primary limiting nutrient. Nutrient additions of N + P (but not N or P singularly) also resulted in significant community shifts, with a strong response by green algae. The management implications for this study are that increases in N and P loading and ratio changes in the lake may result in major phytoplankton community changes toward dominance by green algae. However, increasing P loading relative to N may promote cyanobacterial growth over other phytoplankton groups in this lake system.

The ocean plays a major role in controlling atmospheric carbon at decadal to millennial timescales, with benthic carbon representing the only geologic-scale storage of oceanic carbon. Despite its importance, detailed benthic ocean observations are limited and representation of the benthic carbon cycle in ocean and Earth system models (ESMs) is mostly empirical with little prognostic capacity, which hinders our ability to properly understand the long-term evolution of the carbon cycle and climate change-related feedbacks. The Benthic Ecosystem and Carbon Synthesis (BECS) working group, with the support of the US Ocean Carbon & Biogeochemistry Program (OCB), identified key challenges limiting our understanding of benthic systems, opportunities to act on these challenges, and pathways to increase the representation of these systems in global modeling and observational efforts. We propose a set of priorities to advance mechanistic understanding and better quantify the importance of the benthos: (a) implementing a model intercomparison exercise with existing benthic models to support future model development, (b) data synthesis to inform both model parameterizations and future observations, (c) increased deployment of platforms and technologies in support of in situ benthic monitoring (e.g., from benchtop to field mesocosm), and (d) global coordination of a benthic observing program (“GEOSed”) to fill large regional data gaps and evaluate the mechanistic understanding of benthic processes acquired throughout the previous steps. Addressing these priorities will help inform solutions to both global and regional resource management and climate adaptation strategies.

Turbidity, quantified in turbidity units (nephelometric or formazin), is a common and valid measure of water quality related to transparency. A transparency tube (TT) is an economical tool developed to estimate water clarity as an alternative to the Secchi disk, but it is also frequently used to estimate turbidity. Although the relationship between TT measures and turbidity is well characterized for freshwater river and lake systems, this relationship has not been tested for estuarine waters. The objective of the current study was to empirically determine the TT–turbidity relationship for estuarine waters in coastal South Carolina (SC) and compare these results with the traditional freshwater system conversions. We obtained 107 measurements of TT depth, turbidity, total suspended solids, colored dissolved organic matter absorbance, Secchi depth, and chlorophyll a at 22 estuarine locations in SC over a 1‐year period. Linear regressions provide conversion equations that can be applied to SC estuarine waters. The TT–turbidity relationship for estuaries was compared with freshwater systems. Our results suggest that the slope of the relationship differs between systems (−1.11 vs. −1.41), resulting in different turbidity estimates for TT measurements for estuarine vs. inland waters. We propose a combined conversion table incorporating estuarine and freshwater (riverine and lacustrine) systems. A TT–turbidity conversion for coastal SC and similar estuarine waters significantly benefits current water quality programs and citizen science groups by producing more accurate turbidity estimates for screening and routine monitoring efforts.

Fungi are key drivers of biogeochemical processes, yet marine fungi remain understudied and under-characterized due to primer biases and database gaps. In this study, we conducted a metabarcoding survey targeting the small and large subunit rRNA genes and the internal transcribed spacer region of fungi (18S, 28S, and ITS2) in the sediment and surface water of salt and brackish marshes in the North Inlet-Winyah Bay estuarine system (Georgetown, South Carolina, USA). The universal 18S/16S primer set (515F-Y and 926R) identified few fungal taxa. The ITS2 primer set (ITS3mix and ITS4) revealed high diversity among Dikarya but failed to capture the full extent of early diverging fungi (EDF). In contrast, the 28S primer set (LR0R and LF402) excelled at identifying EDF lineages, including Chytridiomycota, Mucoromycota, Zoopagomycota, and Blastocladiomycota, many of which dominated the brackish marsh sampling site but were less prevalent in the salt marsh sampling sites. Over half of the fungal OTUs identified by the 28S primer set were from EDF lineages. Copy-normalized 28S qPCR showed that EDF were more abundant in brackish sediments than in the salt marsh. Several putative denitrifying fungi, primarily species from Trichoderma and Purpureocillium, were also detected, suggesting overlooked functional guilds that may contribute to estuarine nitrogen cycling. A FUNGuild analysis found that most lineages were saprotrophic. Overall, our findings show that EDF are key contributors to community differences across salinity gradients and may play more important functional roles in coastal biogeochemistry than is currently understood. The 28S primer set is ideal for marine fungal metabarcoding because it provides comprehensive taxonomic coverage and enables phylogenetic analysis.

Phytoplankton community composition in estuaries is tightly linked to freshwater input. While regular freshwater input typically delivers nutrients, fueling phytoplankton growth, the impact of extreme events is less certain. Several observational studies have documented increases in phytoplankton biomass following large precipitation events but cannot to adequately identify the mechanism driving this increase. This paper advances two hypotheses about what drives phytoplankton biomass change following extreme precipitation events. First, the Resident Response Hypothesis (RRH), which suggests local estuarine phytoplankton grow in response to favorable conditions. Alternatively, the Production Introduction Hypothesis (PIH) indicates that large rainfall events introduce new phytoplankton taxa to estuaries during run-off events associated with large storms. These hypotheses were tested at North Inlet Estuary (South Carolina, USA) through a novel experimental design, which utilized multiple treatments to mimic the distinct impacts of large rainfall events on natural phytoplankton communities. Experimental samples were analyzed using photopigments and flow-through imaging microscopy. This allowed a wide assessment of phytoplankton community change both through measuring chlorophyll-a and biomass concentration. Ultimately, there was a strong increase in phytoplankton growth in the storm treatments, primarily identified by diatom pigment increases in response to run-off delivered nutrients, providing support for the RRH. However, biomass concentration analysis of select diatom taxa revealed the introduction of new diatom taxa in run-off water communities. This supports the PIH, suggesting that initial growth from nutrient additions may be due to small cell growth as well as introduction of new taxa. ### Competing Interest Statement The authors have declared no competing interest. Slocum-Lunz Foundation University of South Carolina, https://ror.org/02b6qw903 Yokogawa Fluid Imaging Technologies, Inc.

In marine systems, molar ratios of the biolimiting inorganic nutrients, nitrogen (N) and phosphorus (P), are assumed to follow the “Redfield ratio” (N:P = 16:1), which is often used to infer nutrient limitation of phytoplankton biomass. Traditionally, estuaries are considered N-limited (N:P < 16) but with higher coastal N-loading, reports of P-limitation (N:P > 16) have increased. In recent decades, nutrient loading in North Inlet Estuary (NIE) has changed such that dissolved inorganic nitrogen (DIN):dissolved inorganic phosphorus (DIP) ratios increased from ca. 7 to 20 (2002–2021). This suggests that primary production in NIE may be transitioning from N-limitation to NP co-limitation or primary P-limitation. Due to high N:P ratios, we hypothesized that P would be the primary limiting nutrient for phytoplankton production during the summer of 2023. DIN (20 µmol l−1), low DIP (LP, 5 µmol l−1), and high DIP (HP, 20 µmol l−1), combined DIN + LP, or combined DIN + HP were added to water samples collected monthly at Clambank Landing in NIE and incubated in 48-h bioassays. Changes in phytoplankton biomass (chl a) and community composition were measured via high performance liquid chromatography (HPLC) to determine if enrichment indicated limitation by that nutrient. N was the single or primary limiting nutrient for all bioassays, with potential NP co-limitation in June. Our results suggest that, in NIE, DIN:DIP ratios exceeding 200 may still show phytoplankton N-limitation or NP co-limitation. Ambient nutrient concentrations and ratios are an unreliable indicator of nutrient limitation in this estuary and should not be used to infer limiting nutrients.

Freshwater lakes provide valuable recreational and tourism resources and are a major source of municipal drinking water for local communities. A primary management goal for lake systems is the maintenance of good water quality and a healthy aquatic ecosystem. Blooms of harmful or noxious species of cyanobacteria can result in severe water quality degradation. The purpose of this project was to provide baseline data on phytoplankton community composition, with special emphasis on cyanobacteria, and water quality parameters for Lake Murray, SC. The objective of this study was to determine the annual cyanobacterial contribution to total phytoplankton biomass in the lower reaches of Lake Murray. Measurements for this study were obtained at weekly to biweekly intervals from May 2021 to August 2022 on the northeast side of Lake Murray Dam, SC. Phytoplankton community composition was determined using a combination of high-performance liquid chromatography (HPLC) and ChemTax methods to measure the relative abundances of different algal groups. The phytoplankton community in Lake Murray is composed of a diverse assemblage of primarily green algae, diatoms, cyanobacteria, cryptophytes, and dinoflagellates. Community structure varied seasonally and annually. Total phytoplankton biomass remained below 7 μg of chl a l-1 and never reached “bloom” proportions (e.g., > 40 μg of chl a l-1). Planktonic cyanobacteria were present year-round in the lower reaches of Lake Murray and comprised 5–40% of the total phytoplankton biomass. Peaks in cyanobacteria abundance occurred during the late summer months. Even at the peak in August 2021, concentrations of cyanobacteria were low (1.79 μg chl a l-1). Over annual cycles, cyanobacteria had a median chl a concentration of 0.63 μg l-1 and a median contribution of 14.9%. Although we were unable to identify any specific causal mechanisms for the fluctuations in cyanobacterial biomass, we demonstrated that cyanobacteria are a consistent component of the phytoplankton community in lower Lake Murray. Baseline measures of phytoplankton during “good” water quality conditions provide invaluable data essential for managers to establish criteria for early prediction of bloom events and evaluate the effectiveness of mitigation strategies. Departures from the norm, especially during the summer and early fall, may signal the beginning of a cyanobacterial (or other algal group) bloom and provide an early warning for recreational users and municipal water intakes.

Fungi are key drivers of biogeochemical processes, yet marine fungi remain understudied. While various regions of fungal ribosomal RNA have been targeted to study fungal diversity, the ITS region has been the most prevalent region in the literature since 2012. However, ITS metabarcoding has limitations in marine environments, partly due to database biases. We conducted a metabarcoding survey targeting the small and large subunit rRNA genes and the internal transcribed spacer region of fungi (18S, 28S, and ITS2) in the sediment and surface water of salt and brackish marshes in South Carolina, USA. The 28S primer set (LR0R and LF402) excelled at identifying early diverging fungal lineages, including Chytridiomycota, Mucoromycota, Zoopagomycota, and Blastocladiomycota; however, only the ITS2 primer set amplified Cryptomycota and Olpidiomycota. The universal 18S/16S primer set (515F-Y and 926R) identified few fungal taxa because most reads were prokaryotic. The results based on 28S rRNA amplicons revealed that Dikarya fungi dominated salt marshes, whereas early diverging fungi dominated brackish marshes, suggesting Dikarya are more salt-tolerant. Over half of the fungal OTUs identified by the 28S primer set were from early diverging lineages. A FUNGuild analysis found that saprotrophic fungi are the function of most lineages, but in the brackish marsh, saprotrophic fungi from Zoopagomycota, Blastocladiomycota, and Chytridiomycota were more prevalent. Differential abundance analysis revealed that early diverging fungi were key drivers of community composition between the various marsh types. This study advances our understanding of marine fungal diversity by identifying early diverging lineages that were previously overlooked in marine environments. Our study highlights the vast, unexplored fungal diversity in marine environments.

Climate-induced stressors, such as changes in temperature, salinity, and pH, contribute to the emergence of infectious diseases. These changes alter geographical constraint, resulting in increased Vibrio spread, exposure, and infection rates, thus facilitating greater Vibrio-human interactions. Multiple efforts have been developed to predict Vibrio exposure and raise awareness of health risks, but most models only use temperature and salinity as prediction factors. This study aimed to better understand the potential effects of temperature and pH on V. vulnificus and V. parahaemolyticus planktonic and biofilm growth. Vibrio strains were grown in triplicate at 25°, 30°, and 37°C in 96 well plates containing Modified Seawater Yeast Extract modified with CaCl2 at pH's ranging from 5 to 9.6. AMiGA software was used to model growth curves using Gaussian process regression. The effects of temperature and pH were evaluated using randomized complete block analysis of variance, and the growth rates of V. parahaemolyticus and V. vulnificus were modeled using the interpolation fit on the MatLab Curve Fitting Toolbox. Different optimal conditions involving temperature and pH were observed for planktonic and biofilm Vibrio growth within- and between-species. This study showed that temperature and pH factors significantly affect Vibrio planktonic growth rates and V. parahaemolyticus biofilm formation. Therefore, pH effects must be added to the Vibrio growth modeling efforts to better predict Vibrio risk in estuarine and coastal zones that can potentially experience the cooccurrence of Vibrio and harmful algal bloom outbreak events.

Salt pannes are marsh features in the supratidal zone that are devoid of macrophytic vegetation. Although these habitats appear barren, benthic microalgae (BMA) inhabit the sediments and are potentially important primary producers. In addition, salt pannes are habitats for dense accumulations of sand fiddler crabs ( Leptuca pugilator ; Bosc 1802 ). The purpose of this study was to determine the temporal changes in BMA biomass, community composition, and net primary productivity (NPP) for a supratidal salt panne and quantify sand fiddler crab grazing on BMA. The impact of crab grazing on BMA abundance in surface sediments was determined by measuring chl a concentrations in ungrazed and grazed sediments. BMA biomass peaked to a high of 16 µg chl a g sediment −1 in June and July, suggesting growth in the spring followed by a small decline in the warmer summer months. The BMA community was primarily composed of benthic diatoms, with lesser amounts of cyanobacteria. NPP increased to a median of 0.51 mmol O 2 m −2 h −1 (6.12 mg C m −2 h −1 ) in July. In comparison with other BMA habitats in this estuary, NPP and biomass for salt pannes was lower than the other 5 habitat types (tall and short Spartina , intertidal mud and sandflats, phytoplankton, and submerged sediments). Sand fiddler crabs do not appear to consume significant amounts of BMA during grazing in salt pannes. This first ever study of BMA NPP demonstrates that estuarine salt pannes are likely a small contributor to ecosystem NPP.

We describe a waterproof, lightweight (1.3 kg), low-power (∼1.1 W average power) fluorometer operating on 5 V direct current deployed on a small uncrewed aircraft system (sUAS) to measure chlorophyll and used for triggering environmental water sampling by the sUAS. The fluorometer uses a 450 nm laser modulated at 10 Hz for excitation and a standard photodiode and transimpedance amplifier for the detection of fluorescence. Additional detectors are available for measuring laser intensity and light scattering. Control of the fluorometer and communication between the fluorometer and the Raspberry Pi 4B computer controlling the sampler were provided by an Arduino microcontroller using the robot operating system (ROS). Calibrations were based on standards of dissolved chlorophyll extracted from Chlorella powder (a widely available dietary supplement). The detection limit for chlorophyll from these calibrations was found to be 0.2 μg per liter of water for a single 0.1 s differential measurement. The detection limit decreases with the square root of the integration time as expected. Detection limits increase by a factor of two to three when mounted in the sUAS due to electrical noise; sUAS acoustic noise and vibration do not appear to contribute significantly.

Degradation of estuarine water quality during the Anthropocene has largely resulted from discharges of nutrients leading to eutrophication. Recently, upstream management practices have led to comparatively reduced nutrient input into estuaries. Concurrently, climate cycles and impacts associated with anthropogenic climate warming can affect the long-term conditions observed within estuaries. Using long-term monitoring data from adjacent southeastern U.S. estuaries, we show that decadal-scale trends in nutrient concentrations and phytoplankton standing stock differ between the two connected systems. These contrasting trends appear to result from differences in oceanic influence, the extent of adjacent vegetated marsh, watershed size, and upstream degradation. In the minimally impacted, ocean-dominated North Inlet estuary, we document increasing ammonium and chlorophyll a (Chl a), while in the adjacent, river-dominated Winyah Bay, ammonium, and Chl a concentrations are more variable but do not appear to have increased over the same time period. Surprisingly, total nitrogen exhibits the opposite pattern: temporal stability in North Inlet but increasing in Winyah Bay. We hypothesize that sea level rise associated with climate change has driven a complex set of interactions between salt marsh porewaters and tidal pumping, leading to the spillover of nutrients from salt marshes into tidal creeks in North Inlet. In Winyah Bay, this mechanism is less evident as a driver of ammonium concentrations, likely due to the outsized effect of watershed nutrient input and the narrow fringing marsh platform. The degree to which this mechanism operates in other estuaries, which vary in tidal range, the extent of vegetated marsh, watershed size, and degree of anthropogenic degradation warrants further study.

Cyanobacteria are responsible for the largest number of harmful algal blooms (HABs) worldwide. HABs caused by the genus Microcystis are health threats because they often occur within close proximity to humans and produce phycotoxins such as microcystins that can contaminate drinking water and recreational areas. Molecular techniques enable accurate and rapid (~ 1 h) HAB detection that facilitates monitoring, “early warnings” of blooms, and corresponding management responses. Sandwich hybridization assay (SHA), the technique considered here, directly (no amplification) identifies and quantifies plankton species using ribosomal RNA (rRNA)‐targeted oligonucleotides. This project focused on the development of a new SHA method for the detection of Microcystis (16S rRNA) using laboratory cultures. Assay calibration curve and limits of detection were determined using Microcystis aeruginosa , though signal intensity differed significantly ( p < 0.05) between three species ( M. aeruginosa , Microcystis botrys , Microcystis wesenbergii ). SHA results for M. aeruginosa raised under three light intensities (40, 60, and 100 μ mol photons m −2 s −1 ) and two temperatures (25°C and 32°C) were greatest at the highest irradiance for both temperatures, but otherwise variable, yielding an overall significant ( p < 0.05) interaction between light and temperature. The 32°C treatment also resulted in significantly lower ( p < 0.05) Photosystem II quantum efficiency ( F v / F m ) and microcystins concentrations per light intensity. Spiked field experiments showed that SHA signal was not significantly affected ( p > 0.05) by a mixed phytoplankton assemblage. The assay speed, wide detection range, and specificity indicate that this method has promise for additional field studies and HAB monitoring.

Benthic microalgae (BMA) are an abundant and common component of the illuminated surface sediments of shallow continental shelves worldwide. Although phytoplankton biomass, community composition, and productivity are relatively well documented for these habitats, little is known about the community dynamics of BMA in these systems. We determined the community composition and seasonal abundance of BMA in the surface sediments of shallow (ca. 10-13 m) nearshore shelf waters off Charleston, SC. Our primary hypothesis was that BMA exhibit seasonality in abundance and, similarly, abundance is correlated with bottom water temperatures. BMA had a high spatial variability (CV = 20-50%) over m to km scales. BMA biomass in the upper 1 cm of sediments exceeded phytoplankton by a factor of 3 and was composed primarily of diatoms with cyanobacteria and green microalgae (chlorophytes & euglenophytes) as common, but relatively minor components. The BMA community exhibited a seasonal variation in abundance that was positively correlated with bottom water temperature. The highest abundances occurred in the summer months, peaking in August. This study is the first to report the monthly to annual spatiotemporal dynamics of BMA in the nearshore shelf waters of the South Atlantic Bight and offers insights in the potential contribution of BMA to shelf primary productivity.

Submarine groundwater discharge (SGD) may directly influence the dissolved oxygen (DO) content of coastal bottom waters. Here, we report a predicted episode of enhanced SGD that caused low DO concentrations on the South Carolina continental shelf. The prediction model linked episodes of SGD to upwelling‐favorable winds. The data revealed these waters were a factor of 2–6 higher in 226 Ra and 228 Ra compared to typical bottom water values and were significantly depleted in DO (<130 μM). The tight 228 Ra: 226 Ra correlation of these data was similar to values during a strong hypoxic event off SC in 2012. Water ages from 224 Ra and 223 Ra indicated the event occurred 2–9 days before sampling. The success of the prediction lends added credence to the correlation of upwelling‐favorable winds—but not necessarily accompanied by upwelling—to episodic SGD events. This prediction from wind data represents a major advance for quantifying SGD in the region.

Humans are changing the Earth's surface at an accelerating pace, with significant consequences for ecosystems and their biodiversity. Landscape transformation has far-reaching implications including reduced net primary production (NPP) available to support ecosystems, reduced energy supplies to consumers, and disruption of ecosystem services such as carbon storage. Anthropogenic activities have reduced global NPP available to terrestrial ecosystems by nearly 25%, but the loss of NPP from wetland ecosystems is unknown. We used a simple approach to estimate aquatic NPP from measured habitat areas and habitat-specific areal productivity in the largest wetland complex on the USA west coast, comparing historical and modern landscapes and a scenario of wetland restoration. Results show that a 77% loss of wetland habitats (primarily marshes) has reduced ecosystem NPP by 94%, C (energy) flow to herbivores by 89%, and detritus production by 94%. Our results also show that attainment of habitat restoration goals could recover 12% of lost NPP and measurably increase carbon flow to consumers, including at-risk species and their food resources. This case study illustrates how a simple approach for quantifying the loss of NPP from measured habitat losses can guide wetland conservation plans by establishing historical baselines, projecting functional outcomes of different restoration scenarios, and establishing performance metrics to gauge success.

Nutrient breakpoints are the concentration at which there is a fundamental change in phytoplankton responses to further increases in nutrient loading. The ecological implication is that nutrient breakpoints signal a community transition from a relatively stable state to an alternate or transition state with marked changes in structure and, by extension, function. Nutrient breakpoints for the total phytoplankton community and individual phytoplankton groups in response to increases in dissolved inorganic nitrogen (DIN) concentrations were determined for two contrasting estuarine systems. Phytoplankton were exposed to increasing N addition scenarios (0-100 mu mol DIN L-1) under high and low irradiance conditions. Responses of total algal biomass, group diversity, and individual algal groups were determined after 72-h incubations in experimental bioassays. Phytoplankton from the low-salinity estuary exhibited higher DIN breakpoints (>double) than the high-salinity estuary while irradiance (20% vs. 40% of ambient) did not affect breakpoints. DIN additions changed algal group diversity, but community similarities were > 80%. Our results suggest that ambient DIN concentrations should not exceed breakpoint concentrations to prevent possible shifts from a stable to a transitional or alternate state. DIN thresholds should be 25 and 50 mu mol L(-1)for the high- and low-salinity estuaries, respectively. These levels should mitigate the risk of major alterations in phytoplankton community structure and function in these two estuarine systems.

Water quality management strategies focus primarily on reducing nutrient loading in estuaries to limit total chlorophyll a (chl a) to less than 40 μg l−1. However, potential alterations in phytoplankton community composition and subsequent ecological implications below this limit are not generally considered. The effect of moderate loadings of nitrate (N) and phosphate (P) on nutrient-limited phytoplankton composition and cell size was investigated using multiple bioassays from 2014 to 2016 to evaluate phytoplankton community shifts below the 40 μg l−1 threshold. Water collected from North Inlet Estuary, SC, was spiked with 20 μmol l−1 N and of 10 μmol l−1 P and incubated for 2 days. The proportion of diatoms, cryptophytes, cyanobacteria, prasinophytes, and chlorophytes was calculated for each treatment (i.e., control and NP addition) and for two size fractions (i.e., < 20 μm and whole water). Phytoplankton biomass increases were below the 40 μg chl a l−1 threshold and resulted in a variable shift in phytoplankton group abundances, likely due to the initial community composition. Nutrient additions enhanced the biomass of the fraction smaller than 20 μm and the proportion of diatoms at the expense of cyanobacteria and cryptophytes. Shifts in community composition could have potential cascading impacts on higher trophic levels in the estuary. These results highlight the importance of characterizing and monitoring eutrophication using abundances of algal groups in addition to total biomass as ecologically relevant alterations may occur at low levels of eutrophication.

Carbon concentrating mechanisms (CCMs) are used by phytoplankton to concentrate dissolved inorganic carbon within their cells for use in photosynthesis. However, CCMs which involve carbonic anhydrase (CA) may become redundant in the future due to increasing surface water dissolved CO2 (CO2(aq)) concentrations. Most of our knowledge of the CA enzyme is based on single-species phytoplankton cultures or oligotrophic water samples. Few studies have examined the consequences of CA activity on competitive interactions in estuarine phytoplankton communities or measured the long-term effects on community composition. Using bioassays of natural phytoplankton communities, we explored 2 different estuarine systems and determined how community composition was altered when the CA enzyme was removed. Using the CA inhibitor ethoxyzolamide (EZ), our results demonstrate that communities are altered when the inhibitor is present and CA activity is suppressed. Diatoms were the dominant taxonomic group in all samples following a 3 d exposure of the community to EZ. However, our findings suggest that diatom growth was both stimulated and inhibited, depending on the salinity of the location where samples were collected. Furthermore, microscopy of the high salinity phytoplankton community indicated that centric diatom genera (e.g. Skeletonema, Rhizosolenia) were severely reduced in treatments that removed the competitive advantage of CA, while pennate diatom genera (e.g. Asterionellopsis, Cylindrotheca) dominated these same treatments. These shifts in community structure suggest that phytoplankton composition is affected by carbon acquisition using CA, and some diatom genera may depend on the competitive advantage of this CCM to maintain high abundances in estuarine environments.

Recent studies have focused on carbon concentrating mechanisms and the associated enzyme, carbonic anhydrase, to better understand the efficiency of CO2 uptake rates and carbon fixation in photoautotrophs. Some benthic microalgae (BMA) may be limited by inorganic carbon availability because high photosynthetic rates withdraw a large amount of CO2 and HCO3– in the top layer of sediment. Investigating the mechanisms that affect carbon acquisition are necessary if we are to fully understand the functioning and structuring processes of these systems. From this, we can better predict the potential impacts of increasing atmospheric CO2 concentrations on BMA communities. The purpose of this research was to examine a carbon concentrating mechanism used by BMA through their responses to induced carbon limitation. This approach was conducted through the removal of carbonic anhydrase (CA) activity using an inhibitor, ethoxyzolamide. Microcosm experiments were performed on intertidal muddy sediments from North Inlet Estuary, SC. Exposure to ethoxyzolamide resulted in a reduction of gross primary productivity (GPP) without a reduction in total BMA biomass. Furthermore, removed CA activity caused BMA cumulative GPP maxima to shift upward toward the surface in the sediment column. Active CA was necessary to maintain high GPP rates in these communities and allowed motile BMA to use a wider portion of the sediment column. Available HCO3– at lower depths could still be dehydrated into CO2 by microalgae with CA. Changes in global atmospheric CO2 concentrations leading to higher CO2 availability at the atmosphere-sediment interface may alter the structure and function of these BMA systems, and the vertical distribution of GPP. These consequences may have important implications for the biogeochemical cycling occurring in estuaries.