James L. Pinckney

Phytoplankton diversity is a critical indicator of ecosystem health and function, playing a central role in fueling higher trophic levels and biogeochemical cycling. The Gulf of Maine typically experiences a prominent spring bloom dominated by diatoms. However, in 2023, an extensive and intense bloom of the dinoflagellate, Tripos muelleri, was observed, with concentrations exceeding 50 000 cells L−1, approximately one order of magnitude higher than typical reported abundances,. We used an integrated observational approach combining 18S rRNA amplicon sequencing, pigment analysis and imaging data to characterize bloom structure and dynamics. These datasets were interpreted alongside concurrent oceanographic measurements to evaluate environmental conditions associated with bloom persistence and decline. Results revealed a bloom of T. muelleri that persisted until early August, when an increase in non-viable cells was detected across imaging and pigment-based indicators, coinciding with shifts in environmental conditions. Distance-based redundancy analysis indicated that key environmental variables, including nitrate, phosphate, salinity, temperature and dissolved organic nitrogen, were significantly associated with community composition, collectively explaining 46.5% of the constrained variation across the bloom period. Together, these results highlight the environmental context of an exceptional T. muelleri bloom and underscore the importance of sustained multi-method monitoring to improve understanding of phytoplankton dynamics in the Gulf of Maine.

Contaminants of emerging concern (CECs) are increasingly prevalent in freshwater systems, yet their effects on phytoplankton communities remain poorly understood. We examine how a representative freshwater assemblage responds to four ubiquitous CECs [carbamazepine, diclofenac, 1H,1H,2H,2H-perfluorooctanesulfonic acid (6:2 FTS) and 6ppd-quinone (6PPD-Q)] at environmentally relevant concentrations, monitoring changes in chlorophyll a (chl a), taxonomic composition and photosynthetic efficiency. Bayesian segmented regression (BSR) identified pollutant-specific ecological thresholds (breakpoints). Phytoplankton responses varied markedly among contaminants. 6:2 FTS showed the most consistent above- and below-breakpoint directional slope patterns; most algal groups were modestly stimulated at low concentrations, declining above the threshold. Diclofenac showed tight clustering of algal group breakpoints. Carbamazepine showed dispersed breakpoints, with several sensitive taxa responding above the community-level threshold. 6PPD-Q produced the highest and most variable breakpoints, with significant early declines in chl a. These results suggest that phytoplankton sensitivity to CECs is strongly influenced by contaminant-specific mechanisms and taxa-specific physiology. Chemicals with consistent effects (e.g. 6:2 FTS) can be monitored using community-level metrics, whereas compounds with heterogeneous thresholds (e.g. carbamazepine and 6PPD-Q) require multi-taxa monitoring to detect early ecological changes. Applying BSR to natural communities provides a robust framework for assessing sub-inhibitory impacts of contaminants, thereby enhancing ecological risk assessments.

Bioluminescent bays (biobays) are uncommon coastal ecosystems featuring dense populations of dinoflagellates, often found in mangrove‐ringed lagoons with long water retention times and high organic matter (OM) levels. Although mangroves are associated with high dinoflagellate abundance, it remains uncertain how inorganic nutrients compare with mangrove‐derived organic matter in shaping phytoplankton communities. We performed short‐term in situ bioassays in Mangrove Lagoon, St. Croix, U.S. Virgin Islands, to compare phytoplankton community responses to additions of inorganic nutrients (N and P) and senescent mangrove leaf additions. Phytoplankton composition was assessed using high‐performance liquid chromatography (HPLC) pigment analysis combined with CHEMTAX, and data were analyzed with compositional and centered log‐ratio principal component analyses (CLR–PCAs). Nutrient additions primarily increased total phytoplankton biomass and favored diatom‐dominated communities, consistent with N limitation, but did not significantly alter community composition. Conversely, adding senescent mangrove leaves consistently shifted the community toward higher abundances of peridinin‐containing dinoflagellates, without significantly increasing the abundance of other phytoplankton groups. Multivariate analysis showed that leaf additions shifted the community along a continuous compositional spectrum rather than creating distinct assemblages. These findings suggest that OM (leachate) from senescent mangrove leaves, rather than inorganic N and P enrichment alone, is a plausible mechanism for promoting dinoflagellate dominance in Mangrove Lagoon. Our results underscore the potential role of mangrove OM input in sustaining dinoflagellate‐dominated environments in some tropical lagoons, demonstrating a mechanism by which mangroves may regulate phytoplankton community composition in adjacent waters.

Submarine groundwater discharge (SGD) is a significant source of nutrients to continental shelf waters. Increasing evidence suggests that most of this flow is saline (∼seawater salinity) and occurs across broad continental shelves, making it challenging to observe. Using new, cost‐effective heat‐tracer methods and geochemical analyses of subseafloor saline groundwater, we found that offshore saline SGD delivers repeated pulses of nutrients to the overlying South Atlantic Bight (SAB) water column in the summer. Thermal time‐series measurements were collected below the sandy seafloor 10–15 km offshore of Charleston, SC, during three consecutive summers. Up to seven pulses of saline SGD occurred each summer. Discharge velocities ranged from 1 to 8 cm day ⁻¹ . Pulses of SGD coincided with upwelling‐favorable winds related to annual shifts of the North Atlantic Subtropical High and with winds from storm activity. We observed spatial and temporal variability between SGD pulses from analogous sites 5–10 km apart. Cumulative summertime SGD in 2019 corresponded to 65%–70% of the radium‐based estimates of SGD offshore of South Carolina and was similar in magnitude to the input from regional river discharge. Thermal and geochemical analyses indicate two‐way seawater‐groundwater exchange across the seafloor, reflecting infiltration of seawater into the seafloor and SGD to overlying seawater. Geochemical analyses confirmed high nutrient concentrations in SGD compared to the overlying water column and river water. The δ ¹⁵ N of total dissolved nitrogen and δ ¹³ C of dissolved inorganic carbon in groundwater suggest significant contributions from mineralized organic matter with a geochemical composition distinct from overlying seawater.

Benthic microalgal (BMA) communities contribute significantly to food webs, nutrient cycling, and carbon flows in intertidal habitats. However, the contribution of BMA to saltmarsh carbon stocks (“blue carbon”) is unclear. BMA and sediment total organic carbon (TOC) stocks were measured in an east coast American Atlantic saltmarsh, revealing key relationships between biofilm biomass, carbohydrate, and carbon content. BMA biomass (chlorophyll a ) was highest in Sporobolus stands and mudflat habitats, with diatoms the dominant algal group, and cyanobacteria more important in upper saltmarsh sites. Habitat‐specific differences in biofilm properties (biomass, carbohydrates, photopigments, near‐infrared spectra) corresponded to differences in overall contributions to sediment TOC. Carbohydrates contributed between 8% and 23% of sediment TOC, with the highest levels in Sporobolus and mudflat habitats. BMA biomass and colloidal carbohydrate were significantly correlated, except on lower shore sandflats. The greatest relative contribution of colloidal carbohydrate to %TOC was in upper marsh and tidal channel habitats (1%). Mudflats had the highest %TOC (up to 5% dry weight), but TOC stocks (2000 g C m ⁻² to a depth of 10 cm) were highest in Sporobolus habitats. A modeling approach, based on LIDAR and sediment measures, determined a BMA carbon contribution of 1.3–8% of sediment TOC, with the lowest values in Sporobolus and mudflat habitats. Upscaling from m ² , incorporating habitat heterogeneity, gave median values of 14–16 t TOC ha ⁻¹ for the North Inlet Estuary saltmarshes, of which BMA contributed 0.06–0.08 t C ha ⁻¹ . This approach could permit BMA contributions to blue carbon to be estimated across other saltmarshes.

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.

: 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.

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.