Ths report addresses the following two questions: 
1) What are the loads (flux) of nutrients transported from the Mississippi-Atchafalaya River Basin to the Gulf of Mexico, and where do they come from within the basin? 
2) What is the relative importance of specific human activities, such as agriculture, point-source discharges, and atmospheric deposition in contributing to these loads? 
These questions were addressed by first estimating the flux of nutrients from the Mississippi-Atchafalaya River Basin and about 50 interior basins in the Mississippi River system using measured historical streamflow and water quality data. Annual nutrient inputs and outputs to each basin were estimated using data from the National Agricultural Statistics Service, National Atmospheric Deposition Program, and point-source data provided by the USEPA. Next, a nitrogen mass balance was developed using agricultural statistics, estimates of nutrient cycling in agricultural systems, and a geographic information system. Finally, multiple regression models were developed to estimate the relative contributions of the major input sources to the flux of nitrogen and phosphorus to the Gulf 
of Mexico.

Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin

The oyster population in the Maryland portion of the Chesapeake Bay has declined b y more than 38-fold since the early part of the century. Although the effects of fishing have been implicated, the decline has been attributed primarily to water quality and recently oyster disease. The decline has also been thought to have affected the biota and chemistry of the Bay. Our analysis provides a quantitative demonstration that the long-term declines are largely the result of habitat loss related to over fishing early in the century, stock over fishing, early in the century' through the recent times. Furthermore, the major ecological effects on Chesapeake Bay occurred well-before World War 1/, before industrialization and the prevalence of disease

The decline of Chesapeake Bay oyster population : a century of habitat destruction and overfishing

The mesohaline reach of Chesapeake Bay (USA) recelves most of its allochthonous nutnent input from a single source, the Susquehanna h v e r Seaward of the turb~dity maximum, concentrahons of dissolved inorganic nutrients decrease rapidly as phytoplankton b~omass Increases along the s a l i ~ t y gradient The annual cycle of nverine nutrlent input is In phase with phytoplankton biomass but out of phase with phytoplankton productivity In tfus region kver lne nutnent Input and phytoplankton blomass peak dunng spring but phytoplankton produchvlty peaks dunng summer Seasonal variahons in biomass are correlated \mth rivenne rutrate input whlle seasonal vanations in produchvlty are correlated w t h light and temperature Evldence 1s presented w h ~ c h suggests that the spnng flux of nitrogen from the watershed and the summer productivlty maxlmum are coupled via the accumulahon and sedlmentahon of phytoplankton blomass dunng sprlng and subsequent recycling of regenerated n~trogen Into the euphotic zone during summer We conclude that the occurrence of maxlmum productivity dunng summer in the mesohahne reach of the Bay is a consequence of the recycling of nitrogen dehvered to the system durlng the previous spnng Inter-annual variations In the magmtude of the summer productivlty maximum appear to be related to variahons in vertical stratdicahon which influences the vertical flux of regenerated ammonium from the benthos to the euphotlc zone In this context the extent of seasonal oxygen depletion dunng summer appears to be determined by nverlne mtrate input durlng the spring freshet and the strength and variabihty of vertlcal stratification dunng summer

/pdf/influences-of-river-flow-on-the-dynamics-of-phytoplankton-3cxbe0y6h9.pdf

Influences of river flow on the dynamics of phytoplankton production in a partially stratified estuary

We examined the effects of anthropogenic and climatic perturbations on nutrient-phytoplankton interactions and eutrophication in the waters of the largest estuarine systems in the U.S.A., the Chesapeake Bay (CB), Maryland/ Virginia, and the Neuse River Estuary/Pamlico Sound (NRE/PS) system, North Carolina. Both systems have experienced large post-World War II increases in nitrogen (N) and phosphorus (P) loading, and nutrient reductions have been initiated to alleviate symptoms of eutrophication. However, ecosystem-level effects of these nutrient reductions are strongly affected by hydrologic variability, including severe droughts and a recent increase in Atlantic hurricane activity. Phytoplankton community responses to these hydrologic perturbations, including storm surges and floods, were examined and when possible, compared for these systems. In both systems, the resulting variability in water residence time strongly influenced seasonal and longer-term patterns of phytoplankton biomass and community composition. Fast-growing diatoms were favored during years of high discharge and short residence time in CB, whereas this effect was not observed during high discharge conditions in the longer residence time NRE/ PS. In the NRE/PS, all phytoplankton groups except summer cyanobacterial populations showed decreased abundance during elevated flow years when compared to low flow years. Although hurricanes affected the CB less frequently than the NRE/PS, they nonetheless influenced floral composition in both systems. Seasonally, hydrologic perturbations, including droughts, floods, and storm-related deep mixing events, overwhelmed nutrient controls on floral composition. This underscores the difficulty in predicting seasonal and longer-term phytoplankton production and compositional responses to nutrient input reductions aimed at controlling eutrophication of large estuarine ecosystems.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2006.51.1_part_2.0448

Anthropogenic and climatic influences on the eutrophication of large estuarine ecosystems

Sediment-water oxygen and nutrient (NH4
 +, NO3
 −+NO2
 −, DON, PO4
 3−, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l−1, apparently limiting SOC rates. Fluxes of NH4
 + were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations ( 500 μmol N m−2 h−1) were observed. Nitrate + nitrite (NO3
 −+NO2
 −) exchanges were directed into sediments in areas where bottom water NO3
 −+NO2
 − concentrations were high (>18 μM N); sediment efflux of NO3
 −+NO2
 − occurred only in areas where bottom water NO3
 −+NO2
 − concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m−2 h−1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH4
 −, PO4
 3−, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.

Sediment-water oxygen and nutrient exchanges along the longitudinal axis of Chesapeake Bay: Seasonal patterns, controlling factors and ecological significance

Spatial and temporal aspects of the coupling of nutrient inputs to estuarine primary production are discussed in terms of formation, transport and decomposition of particulate material in the Patuxent River estuary. Mixing diagrams of dissolved and particulate nitrogen and phosphorus (DIN, PN, DIP, PP) versus salinity indicate rapid removal of DIN and DIP with parallel formation of PN and PP in the slightly brackish (0-5%0) region. However, plankton production (per m2) tends to be inversely related to salinity between 0 and 10%0. While most of the annual nutrient inputs to the Patuxent occur between December and April, peak plankton production occurs in the summer. Deposition and resuspension of particulate substances in this system are dynamic processes which decrease in a seaward direction. Regeneration of DIN and DIP from particulate decomposition in the sediments similarly decreases toward the estuary's mouth; however, highest rates are in the 5-100/00 region. Temporal patterns of benthic regeneration and plankton production are congruent (being generally related to temperature), and rates of sediment NIL, + recycling represent about 60-80% of the plankton assimilative demand in the 5-100/00 region. On the basis of these observations we propose a conceptual model in which annual inputs of dissolved nutrients from runoff are converted into particulate forms which are gradually transported seaward through cycles of deposition and resuspension. As temperatures rise in the summer these particulates are decomposed, releasing dissolved nutrients to the euphotic zone where they support the major annual blooms of plankton. By comparing these patterns for the Patuxent with those reported for other estuaries, it is suggested that this model may be relatively general for many coastal

W. M. Kemp

Papers

Spatial and temporal coupling of nutrient inputs to estuarine primary production: the role of particulate transport and decomposition