Long-Term Trends of Nutrients and Phytoplankton in Chesapeake Bay

Abstract

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.

The competing impacts of climate change and nutrient reductions on dissolved oxygen in Chesapeake Bay

“…In the future, precipitation is most likely to increase most during the winter/spring and in the northern part of the region (Najjar et al, 2009;IPCC Annex I, 2013), delivering higher river flows and nutrient loads that fuel spring productivity and produce more organic matter available for summer decomposition . Changes in nutrient loading and hydrologic conditions can also alter the bay's phytoplankton composition, changing the biomass available for eventual decomposition (Harding et al, 2016).…”

The competing impacts of climate change and nutrient reductions on dissolved oxygen in Chesapeake Bay

“…In the future, precipitation is most likely to increase most during the winter/spring and in the northern part of the region (Najjar et al, 2009;IPCC Annex I, 2013), delivering higher river flows and nutrient loads that fuel spring productivity and produce more organic matter available for summer decomposition (Najjar et al, 2010). Changes in nutrient 60 loading and hydrologic conditions can also alter the Bay's phytoplankton composition, changing the biomass available for eventual decomposition (Harding et al, 2015(Harding et al, , 2016.…”

Challenges associated with modeling low-oxygen waters in Chesapeake Bay: a multiple model comparison

“…Although hypoxia in the Chesapeake Bay has likely existed since European colonization Brush, 1991, 1993), recent studies have highlighted an accelerated rise in the number and spatial extent of hypoxic, as well as anoxic (DO concentrations < 0.2 mg L −1 ), events in the bay since the 1950s, primarily attributed to increased anthropogenic nutrient input (Hagy et al, 2004;Kemp et al, 2005;Gilbert et al, 2010). These impacts are likely to be exacerbated by future climate change (Najjar et al, 2010;Meire et al, 2013;Harding Jr. et al, 2015).…”