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.

Keywords: ecosystem management; eutrophication; global change; seagrass; submersed aquatic vegetation.

Fig. 1.

Structural equation models for total nitrogen (N) fit to subestuaries and their watersheds by salinity zone. (A) Tidal freshwater/oligohaline (0–5 psu), (B) mesohaline (5–15 psu), and (C) polyhaline (15–25 psu). Arrow width is proportional to the standardized effect size, given next to the arrows. Black arrows denote positive effects; red arrows, negative effects. Nonsignificant relationships (P > 0.05) have been omitted for clarity, including the nonsignificant effects of point source nutrients and total suspended solids (TSS) (SI Appendix, Fig. S5). Map Insets denote the location of watersheds. Units and unstandardized path coefficients are given in SI Appendix, Supplementary Materials.

Fig. 2.

Bay-wide structural equation model representing the effects of water quality on SAV cover. Arrow width is proportional to the standardized effect size, given next to the arrows. Black arrows denote positive effects; red arrows, negative effects. Nonsignificant relationships (P > 0.05) have been omitted for clarity, including the nonsignificant effects of total suspended solids (TSS) (SI Appendix, Fig. S6). Units and unstandardized path coefficients are given in SI Appendix, Supplementary Materials.

Fig. 3.

Annual bay-wide trends, and trends by salinity zone, in (A) total observed SAV cover (hectares, from aerial monitoring survey), (B) mean water column nitrogen, and (C) mean water column phosphorus concentrations (milligrams per liter, from in situ water quality monitoring). SAV cover was not obtained for 1 y (1988).