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The use of nutrients by diverse phytoplankton communities in estuarine systems, and their response to changes in physical and biogeochemical processes in these natural systems, is a significant ongoing area of research. We used a whole ecosystem 15NO3− tracer experiment to determine the uptake of different nitrogen (N) forms in phytoplankton functional groups over a mid- to neap tidal cycle in a salt marsh creek in Plum Island Estuary, Massachusetts, USA. We quantified the biomass and δ15N for three groups corresponding to micro- (20–200 μm; microP), nano- (3–20 μm; nanoP), and picophytoplankton (< 3 μm; picoP). All three size classes showed distinct use of recycled N sources throughout the 11-day sampling period and minimal direct assimilation of the 15NO3− tracer. MicroP consistently used high amounts of creek-derived 15NH4+, even with a shift at neap tide from diatom- to dinoflagellate-dominated communities (including members of the harmful genus Alexandrium). NanoP use of recycled 15NH4+ increased over the mid-neap tidal cycle, while picoP use decreased. Both biomass and NH4+ use (as highest δ15N values) of all size groups were maximized during neap tide. This study demonstrates partitioning of recycled N use among size-based phytoplankton groups in the estuary, with distinct effects of tidal cycle on the nutrient uptake of each group, and with important implications for the roles of diverse phytoplankton communities in estuarine nutrient cycling. 

Intensive agriculture alters headwater streams, but our understanding of its effects is limited in tropical regions where rates of agricultural expansion and intensification are currently greatest. Riparian forest protections are an important conservation tool, but whether they provide adequate protection of stream function in these areas of rapid tropical agricultural development has not been well studied. To address these gaps, we conducted a study in the lowland Brazilian Amazon, an area undergoing rapid cropland expansion, to assess the effects of land use change on organic matter dynamics (OM), ecosystem metabolism, and nutrient concentrations and uptake (nitrate and phosphate) in 11 first order streams draining forested (n = 4) or cropland (n = 7) watersheds with intact riparian forests. We found that streams had similar terrestrial litter inputs, but OM biomass was lower in cropland streams. Gross primary productivity was low and not different between land uses, but ecosystem respiration and net ecosystem production showed greater seasonality in cropland streams. Although we found no difference in stream concentrations of dissolved nutrients, phosphate uptake exceeded nitrate uptake in all streams and was higher in cropland than forested streams. This indicates that streams will be more retentive of phosphorus than nitrogen and that if fertilizer nitrogen reaches streams, it will be exported in stream networks. Overall, we found relatively subtle differences in stream function, indicating that riparian buffers have thus far provided protection against major functional shifts seen in other systems. However, the changes we did observe were linked to watershed scale shifts in hydrology, water temperature, and light availability resulting from watershed deforestation. This has implications for the conservation of tens of thousands of stream kilometers across the expanding Amazon cropland region. 

Abstract Excess reactive nitrogen (N) flows from agricultural, suburban, and urban systems to coasts, where it causes eutrophication. Coastal wetlands take up some of this N, thereby ameliorating the impacts on nearshore waters. Although the consequences of N on coastal wetlands have been extensively studied, the effect of the specific form of N is not often considered. Both oxidized N forms (nitrate, NO3−) and reduced forms (ammonium, NH4+) can relieve nutrient limitation and increase primary production. However, unlike NH4+, NO3− can also be used as an electron acceptor for microbial respiration. We present results demonstrating that, in salt marshes, microbes use NO3− to support organic matter decomposition and primary production is less stimulated than when enriched with reduced N. Understanding how different forms of N mediate the balance between primary production and decomposition is essential for managing coastal wetlands as N enrichment and sea level rise continue to assail our coasts.