This review provides an integrated synthesis with timelines and evaluations of ecological responses to eutrophication in Chesapeake Bay, the largest estuary in the USA. Analyses of dated sediment cores reveal initial evidence of organic enrichment in ~200 yr old strata, while signs of increased phytoplankton and decreased water clarity first appeared ~100 yr ago. Severe, recurring deep-water hypoxia and loss of diverse submersed vascular plants were first evident in the 1950s and 1960s, respectively. The degradation of these benthic habitats has contributed to declines in benthic macroinfauna in deep mesohaline regions of the Bay and blue crabs in shallow polyhaline areas. In contrast, copepods, which are heavily consumed in pelagic food chains, are relatively unaffected by nutrient-induced changes in phytoplankton. Intense mortality associated with fisheries and disease have caused a dramatic decline in eastern oyster stocks and associated Bay water filtration, which may have exacerbated eutrophication effects on phytoplankton and water clarity. Extensive tidal marshes, which have served as effective nutrient buffers along the Bay margins, are now being lost with rising sea level. Although the Bay's overall fisheries production has probably not been affected by eutrophication, decreases in the relative contribution of demersal fish and in the efficiency with which primary production is transferred to harvest suggest fundamental shifts in trophic and habitat structures. Bay ecosystem responses to changes in nutrient loading are complicated by non-linear feedback mechanisms, including particle trapping and binding by benthic plants that increase water clarity, and by oxygen effects on benthic nutrient recycling efficiency. Observations in Bay tributaries undergoing recent reductions in nutrient input indicate relatively rapid recovery of some ecosystem functions but lags in the response of others. KEY WORDS: Eutrophication · Nutrients · Chesapeake Bay Resale or republication not permitted without written consent of the publisherChesapeake Bay is a large estuary which has undergone many changes in its ecological properties and processes in response to nutrient enrichment over the last 2 centuries. Susceptibility of the Bay to eutrophication arises in part from the long dendritic shoreline that intimately connects it to its large watershed (covering an area 15 times that of the Bay) which contains expanding human population centers and extensive agricultural activities. (Satellite image from MODIS,

Climate effects on hydrology impart high variability to water-quality properties, including nutrient loadings, concentrations, and phytoplankton biomass as chlorophyll-a (chl-a), in estuarine and coastal ecosystems. Resolving longterm trends of these properties requires that we distinguish climate effects from secular changes reflecting anthropogenic eutrophication. Here, we test the hypothesis that strong climatic contrasts leading to irregular dry and wet periods contribute significantly to interannual variability of mean annual values of water-quality properties using in situ data for Chesapeake Bay. Climate effects are quantified using annual freshwater discharge from the Susquehanna River together with a synoptic climatology for the Chesapeake Bay region based on predominant sea-level pressure patterns. Time series of waterquality properties are analyzed using historical and recent data for the bay adjusted for climate effects on hydrology. Contemporary monitoring by the Chesapeake Bay Program (CBP) provides data for a period since mid-1984 that is significantly impacted by anthropogenic eutrophication, while historical data back to 1945 serve as historical context for a period prior to severe impairments. The generalized additive model (GAM) and the generalized additive mixed model (GAMM) are developed for nutrient loadings and concentrations (total nitrogen-TN, nitrate +nitrate-NO 2 +NO 3 ) at the Susquehanna River and waterquality properties in the bay proper, including dissolved nutrients (NO 2 +NO 3 , orthophosphate-PO 4 ), chl-a, diffuse light attenuation coefficient (K D (PAR)), and chl-a/TN. Each statistical model consists of a sum of nonlinear functions to generate flow-adjusted time series and compute long-term trends accounting for climate effects on hydrology. We present results identifying successive periods of (1) eutrophication ca. 1945-1980 characterized by approximately doubled TN and NO 2 +NO 3 loadings, leading to increased chl-a and associated ecosystem impairments, and (2) modest decreases of TN and NO 2 +NO 3 loadings from 1981 to 2012, signaling a partial reversal of nutrient over-enrichment. Comparison of our findings with longterm trends of water-quality properties for a variety of estuarine and coastal ecosystems around the world reveals that trends for Chesapeake Bay are weaker than for other systems subject to strenuous management efforts, suggesting that more aggressive actions than those undertaken to date will be required to counter anthropogenic eutrophication of this valuable resource.

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Variable climatic conditions strongly influence phytoplankton dynamics in estuaries globally. Our study area is Chesapeake Bay, a highly productive ecosystem providing natural resources, transportation, and recreation for nearly 16 million people inhabiting a 165,000-km2 watershed. Since World War II, nutrient over-enrichment has led to multiple ecosystem impairments caused by increased phytoplankton biomass as chlorophyll-a (chl-a). Doubled nitrogen (N) loadings from 1945–1980 led to increased chl-a, reduced water clarity, and low dissolved oxygen (DO), while decreased N loadings from 1981–2012 suggest modest improvement. The recent 30+ years are characterized by high inter-annual variability of chl-a, coinciding with irregular dry and wet periods, complicating the detection of long-term trends. Here, we synthesize time-series data for historical and recent N loadings (TN, NO2 + NO3), chl-a, floral composition, and net primary productivity (NPP) to distinguish secular changes caused by nutrient over-enrichment from spatio-temporal variability imposed by climatic conditions. Wet years showed higher chl-a, higher diatom abundance, and increased NPP, while dry years showed lower chl-a, lower diatom abundance, and decreased NPP. Our findings support a conceptual model wherein variable climatic conditions dominate recent phytoplankton dynamics against a backdrop of nutrient over-enrichment, emphasizing the need to separate these effects to gauge progress toward improving water quality in estuaries.

Interannual variability of the spring phytoplankton bloom is strongly expressed in estuarine ecosystems such as Chesapeake Bay. Quantifying this variability is essential to resolve ecosystem responses to eutrophication from variability imposed by climate. We developed a 'synoptic climatology' from surface sea-level pressure (SLP) maps to categorize and quantify atmospheric circulation patterns and address climate forcing of phytoplankton dynamics in the Bay. The 10 patterns we identified had unique frequencies-of-occurrence and associated meteorological conditions (i.e. precipitation, temperature, wind speed and direction). Four measures of phytoplankton biomass, surface chlorophyll a (B), euphotic layer chlorophyll a (B eu ), water column chlorophyll a (B wc ), and total biomass (B tot ), were obtained from remotely sensed ocean color data spanning 16 yr (1989 to 2004) combined with concurrent shipboard data. Years with more frequent warm/wet weather patterns had spring blooms that reached peak biomass farther seaward in the estuary, were greater in magnitude, occurred later in the spring, and covered a larger area than years with a predominance of cool/dry weather patterns. Frequencies of winter weather patterns were used to forecast spring B, B eu , B wc , and B tot , explaining between 23 and 89% of the variance in the regional time-series. Residuals from these models did not show time-trends attributable to either accelerating eutrophication or management actions intended to decrease nutrient loadings. These findings extend our understanding of climatic influences on phytoplankton dynamics in the Bay by quantifying the effects of synoptic climate variability on spring bloom intensity, thereby supporting forecasts of seasonal phytoplankton biomass based on sub-continental scale weather patterns in this mid-Atlantic estuary. KEY WORDS: Spring bloom · Phytoplankton dynamics · Synoptic climatology · Chesapeake BayResale or republication not permitted without written consent of the publisher OPEN PEN ACCESS CCESS