Abstract
Coincident climatic and human effects strongly influence water-quality properties in estuarine-coastal ecosystems around the world. Time-series data for a number of ecosystems reveal high spatio-temporal variability superimposed on secular trends traceable to nutrient over-enrichment. In this paper, we present new analyses of long-term data for Chesapeake Bay directed at several goals: (1) to distinguish trends from spatio-temporal variability imposed by climatic effects; (2) to assess long-term trends of water-quality properties reflecting degradation and recovery; (3) to propose numerical water-quality criteria as targets for restoration; (4) to assess progress toward attainment of these targets. The bay has experienced multiple impairments associated with nutrient over-enrichment since World War II, e.g., low dissolved oxygen (DO), decreased water clarity, and harmful algal blooms (HAB). Anthropogenic eutrophication has been expressed as increased chlorophyll-a (chl-a) driven by accelerated nutrient loading from 1945 to 1980. Management intervention led to decreased loading thereafter, but deleterious symptoms of excess nutrients persist. Climatic effects exemplified by irregular “dry” and “wet” periods in the last 30+ years largely explain high inter-annual variability of water-quality properties, requiring adjustments to resolve long-term trends. Here, we extend these analyses at a finer temporal scale to six decades of chl-a, Secchi depth, and nitrite plus nitrate (NO2 + NO3) data to support trend analyses and the development of numerical water-quality criteria. The proposed criteria build on a conceptual model emphasizing the need to distinguish climatic and human effects in gauging progress to reverse eutrophication in estuarine-coastal ecosystems.
Highlights
Significant changes in estuarine-coastal ecosystems around the world can be traced to climatic and anthropogenic effects[1,2,3,4]
Historical records for Susquehanna River flow (SRF) showed high inter-annual variability from 1960 to 2015, the period corresponding to observations of water-quality properties analyzed here (Fig. 2)
Numerical water-quality criteria were developed for Chesapeake Bay using statistical models to adjust for climatic effects
Summary
Significant changes in estuarine-coastal ecosystems around the world can be traced to climatic and anthropogenic effects[1,2,3,4]. Recent analyses of multi-decadal time series have proven effective to identify secular changes for a diverse set of estuarine-coastal ecosystems, including Narragansett Bay in Rhode Island (USA)[8], Chesapeake Bay in the mid-Atlantic (USA)[9], the Neuse River estuary in North Carolina (USA)[10], Tampa Bay in Florida (USA)[11], the San Francisco Bay estuary in California (USA)[12], the Baltic Sea in northern Europe[13], and the northern Adriatic Sea in southern Europe[14] These studies and others supported a global synthesis for ecosystems at the land-sea margin, focusing on long-term trends and major drivers of spatio-temporal variability[5]. Consistent with the common use of annual means to define overall patterns and trends[5], we documented climatic and human effects on phytoplankton dynamics in the bay using mean, annual chl-a, net primary productivity (NPP), cell-size distribution, and floral composition[17,18,24]
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