Long-term nutrient reductions lead to the unprecedented recovery of a temperate coastal region

Abstract

Humans strongly impact the dynamics of coastal systems, yet surprisingly few studies mechanistically link management of anthropogenic stressors and successful restoration of nearshore habitats over large spatial and temporal scales. Such examples are sorely needed to ensure the success of ecosystem restoration efforts worldwide. Here, we unite 30 consecutive years of watershed modeling, biogeochemical data, and comprehensive aerial surveys of Chesapeake Bay, United States to quantify the cascading effects of anthropogenic impacts on submersed aquatic vegetation (SAV), an ecologically and economically valuable habitat. We employ structural equation models to link land use change to higher nutrient loads, which in turn reduce SAV cover through multiple, independent pathways. We also show through our models that high biodiversity of SAV consistently promotes cover, an unexpected finding that corroborates emerging evidence from other terrestrial and marine systems. Due to sustained management actions that have reduced nitrogen concentrations in Chesapeake Bay by 23% since 1984, SAV has regained 17,000 ha to achieve its highest cover in almost half a century. Our study empirically demonstrates that nutrient reductions and biodiversity conservation are effective strategies to aid the successful recovery of degraded systems at regional scales, a finding which is highly relevant to the utility of environmental management programs worldwide.

Value of ecosystem-based management

“…The results of Lefcheck et al (1) show the nutrient reduction enabled by a series of watershed scale agreements over the last three decades are consistent with the observed increase in the bay-wide area of underwater grasses, also known as submersed aquatic vegetation (SAV). SAV is one of the primary sentinels of bay health, and the extent of eelgrass (Zostera marina) and the 15 other species commonly found in different parts of the bay is one of the primary means by which scientists evaluate the effectiveness of nutrient reduction measures and other restoration strategies (2).…”

“…At 195 miles long and 3,237 square miles in area, the size and complexity of the bay's coupled social and ecological systems has challenged efforts to assess its health. Building on more than 30 y of research, Lefcheck et al (1) offer an innovative way to take the pulse of this much loved and debilitated coastal estuary. By developing a model of the bay ecosystem that brings data from longterm, large-scale monitoring together with knowledge of the mechanisms underlying those patterns, Lefcheck et al make a compelling case that ecosystem restoration initiatives of the last 30+ y are paying off.…”

“…SAV is one of the primary sentinels of bay health, and the extent of eelgrass (Zostera marina) and the 15 other species commonly found in different parts of the bay is one of the primary means by which scientists evaluate the effectiveness of nutrient reduction measures and other restoration strategies (2). Based on their results, Lefcheck et al (1) propose that much of the SAV recovery observed through aerial and onthe-ground surveys over the last 30 y is a direct result of the monumental effort to reduce nutrient inputs and lower nutrient concentrations throughout the bay.…”

“…Lefcheck et al (1) sought to trace the within-ecosystem and ecosystem-human system connections that are most salient to explaining changes in the extent of underwater grasses throughout the bay. These grasses are both beneficiaries and enablers of ecosystem recovery (2).…”

Phosphorus and the Chesapeake Bay: Lingering Issues and Emerging Concerns for Agriculture

“…As predicted by the version of the Chesapeake Bay Watershed Model that was used at the TMDL midpoint (Phase 5.3.2), total P loads to the bay from activities related to major nonpoint sources of P (urban runoff, agricultural sources, and natural areas) have dropped by 16% (1.12 million kg total P) from 2009 to 2017 (Chesapeake Bay Program, 2018aProgram, , 2018b. Concurrently, early signs of improvements to the ecological health of the Chesapeake Bay have also been reported, including increased coverage of submerged aquatic vegetation and higher dissolved oxygen levels (Lefcheck et al, 2018;Zhang et al, 2018). As the TMDL proceeds into its later stages, challenges remain in the area of reducing nonpoint source loads from agricultural and suburban landscapes.…”