Estuaries receive continuous inputs of nutrients from their freshwater sources, but the fate of the inputs is poorly known. In order to document nutrient removal from the water column by phytoplankton, we measured the distributions of turbidity, nutrients, and phytoplankton across the salinity gradients of three estuaries: Chesapeake Bay, Delaware Bay, and the Hudson river estuary. Mixing diagrams were used to distinguish between conservative and non-conservative behavior; i.e. between loss from the water column and export to the estuarine plume on the shelf. In Chesapeake and Delaware Bays, we frequently observed a turbidity maximum in the oligohaline region, a chlorophyll maximum in clearer waters seaward of the turbidity maximum, and a nutrient-depleted zone at the highest salinities. In the Hudson River estuary, mixing diagrams were dominated by lateral waste inputs from New York City, and nutrient removal could not be estimated. In Chesapeake Bay, there was consistent removal of total N, nitrate, phosphate, and silicate from the water column, whereas in Delaware Bay, total N, ammonium, total P, and phosphate were removed. Total N and P removal in the Chesapeake and Delaware are estimated as ca. 50%, except for TP in the Chesapeake, which appeared to be conservative. Phytoplankton accumulation was associated with inorganic nutrient removal, suggesting that phytoplankton uptake was a major process responsible for nutrient removal. In the high salinity zone near and in the shelf plume, an index of nutrient limitation suggested no limitation in the Hudson, slight or no limitation in the Delaware, and widespread limitation in the Chesapeake, especially for P. These observations and information from the literature are summarized as a conceptual model of the chemical and biological structure of estuaries.

Phytoplankton, nutrients, and turbidity in the Chesapeake, Delaware, and Hudson estuaries

During a 1-year period we measured discharges of water, suspended solids, and nutrients from 27 watersheds having differing proportions of cropland in the Piedmont and Coastal Plain provinces of the Chesapeake Bay drainage. Annual flow-weighted mean concentrations of nitrate and organic N and C in stream water correlated with the relative proportions of base flow and storm flow. As the proportion of base flow increased, the concentration of nitrate increased and the concentrations of organic N and C decreased. This suggests that discharge of nitrate is promoted by groundwater flow but discharges of organic N and C are promoted by surface runoff. Concentrations of N species also increased as the proportion of cropland increased. We developed a statistical model that predicts concentrations of N species from the proportions of cropland and base flow. P concentrations did not correlate with cropland or base flow but correlated with the concentration of suspended solids, which differed among watersheds.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97WR02005

Relating nutrient discharges from watersheds to land use and streamflow variability

A first look at the distribution of the stable isotopes of silicon in natural waters

Mechanistic and empirical models were used to examine relationships between primary production and environmental variables along the Mississippi River plume/oceanic gradient off Southwest Pass, Louisiana. A large proportion of variation in primary production could be explained on the basis of light and biomass ( r 2 > 0.857, N > 25). However, comparison of observed chlorophyll concentrations with those predicted using a steady-state light limitation model suggested factors in addition to light availability constrained maximum biomass levels in the plume. Factors which may have contributed to low observed biomass included growth limitation or inhibition by substances not measured, losses due to grazing and sinking, and a short residence time for plume waters, which may have prevented populations from reaching steady state. The dissipative effects associated with plume/oceanic mixing may have been enhanced by potential inhibitory effects of large and varying salinity gradients. Nutrient-salinity distributions, in conjunction with approximate calculations of primary consumption of riverine nutrient sources by phytoplankton, led to the conclusion that biomass and production were controlled by nutrient supply at salinities above 30.

Enhanced primary production at the plume/oceanic interface of the Mississippi River

Seasonal variations in the distribution of dissolved inorganic nitrogen, sihcon, and phosphorus along the salinity gradient of Chesapeake Bay from 1984 to 1988 suggest that dissolved silicate (DSi) controls the magnitude of diatom production during the spring bloom, causes the collapse of the spring bloom, and leads to changes in floristic composition. High sedimentation rates of chlorophyll biomass observed during this per~od could be due to Si-deficiency, suggesting that the supply of DSi may also control the flux of phytoplankton biomass to the benthos, an important parameter of seasonal oxygen depletion in the Bay.

/pdf/annual-cycle-of-dissolved-silicate-in-chesapeake-bay-1798oge4k2.pdf

Annual cycle of dissolved silicate in Chesapeake bay: implications for the production and fate of phytoplankton biomass

Monthly data collected in 1981 from the James, York and Rappahannock rivers show that dissolved silica (= silicic acid) exhibits two distinct types of longitudinal distributions in these estuaries during the year. Conservative behavior was observed during winter or when river discharge was high. The second distribution occurs when uptake by planktonic diatoms reduces concentrations below the levels attributable to dilution by seawater. In all three systems, the non-conservative behavior is characterized by rapid uptake of silicic acid in the tidal freshwater portion, just upstream of the limit of salt intrusion, and corresponds to a chlorophyll maximum shown to contain bloom densities of diatoms. Silicic acid is exhausted below detectable concentrations in the vicinity of the diatom maximum, representing more than 98% removal of the riverine source. Non-conservative distributions of total silica (= silicic acid + biogenic silica) in this same region indicate deposition of biogenic silica to the sediments. At the same time the riverine source is depleted by the diatoms in the freshwater reaches, silicic acid is resupplied to the water column in the oligohaline portion of the estuaries. This resupply is shown to be significant, nearly equal to the amount of silicic acid entering the estuary in river input, suggesting complete mineralization of biogenic silica produced in the freshwater zone. The spatial distributions described above were observed regularly in all three systems, indicating that enhanced phytoplankton production and nutrient regeneration in the vicinity of the transition zone is a natural feature of these partially-mixed estuarine rivers. Possible mechanisms contributing to the accumulation of phytoplankton biomass and the enhanced cycling of silicon are discussed.

Gary F. Anderson

Papers

Silica, diatoms and a freshwater productivity maximum in Atlantic Coastal Plain estuaries, Chesapeake Bay☆