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Abstract 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. 

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 O2 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. 

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.