Fish declines linked to effects of excess nutrients on coastal estuaries

Excess nutrients stimulate the growth of algae in Elkhorn Slough, leading to the formation of green algal mats on the surface. Credit: Brent Hughes

Fish declines linked to effects of excess nutrients on coastal estuaries

Posted: 08 Jun 2015 06:31 PM PDT

A comprehensive study of a major California estuary has documented the links between nutrient runoff from coastal land use, the health of the estuary as a nursery for young fish, and the abundance of fish in an offshore commercial fishery. The study, published the week of June 8, 2015, in Proceedings of the National Academy of Sciences, focused on Elkhorn Slough and Monterey Bay on California’s central coast. Lead author Brent Hughes, now a postdoctoral research fellow at the University of California, Santa Cruz, began studying water quality in Elkhorn Slough as a UCSC graduate student. His earlier research showed that virtually every portion of the estuary is adversely affected by high nutrient levels, which stimulate the growth of algae, leading to low oxygen levels when the algae die and decompose. The new study, based on data collected over the past 40 years, shows how low levels of dissolved oxygen (a condition known as “hypoxia”) affects fish populations in the estuary and beyond. “We found that declines in dissolved oxygen levels were consistently associated with declines in the diversity and abundance of fish in Elkhorn Slough,” Hughes said. “In particular, we saw a drop in certain species of fish that we know use the estuary as a nursery ground for juveniles.” English sole is one of the fish species that uses Elkhorn Slough as a nursery, and the study found that low oxygen was associated not only with fewer juveniles in the estuary, but also with later declines in the numbers of English sole caught in the commercial fishery and scientific fish surveys in Monterey Bay. “From a conservation perspective, these findings suggest that improvements in land management and reductions in nutrient runoff could directly benefit estuaries and indirectly benefit offshore fisheries due to the important role of estuaries as nurseries for some species,” said coauthor Mary Gleason, lead marine scientist for The Nature Conservancy in California…..


The researchers found that climate can be a powerful moderator of coastal hypoxia, but the effects of climate change are hard to predict. Climate change is likely to bring warmer water temperatures in the future, and warm water holds less oxygen than cold water. It is also likely to influence the frequency and intensity of El Niño events, but exactly how is still unclear. While more research is needed to develop predictive models of how climate change will affect coastal ecosystems, reducing the stress caused by nutrient enrichment and hypoxia will improve the resilience of those ecosystems and help buffer the effects of climate change, Hughes said.

According to Gleason, there are many opportunities for improving coastal land management practices. “This study demonstrates the important connections between land management and the health of our estuaries and oceans. We need to have better dialog between land managers and ocean managers–between farmers and fishermen–to ensure that land use practices are being improved to reduce adverse effects of nutrients on coastal and marine ecosystems,” she said. In the area around Elkhorn Slough, The Nature Conservancy is working with partners to reduce nutrient runoff through improved land management practices and treatment wetlands. “Poor water quality is an important threat to some offshore fisheries and marine biodiversity, so it’s critical to improve land-based management and reduce runoff,” said Gleason. “In addition to managing nutrient applications, another solution is to restore or build wetlands at the lower end of drainage systems from farms. These treatment wetlands can collect, filter, and treat agriculture discharge water to reduce nutrient levels before the water enters the estuary.”



Brent B. Hughes, Matthew D. Levey, Monique C. Fountain, Aaron B. Carlisle, Francisco P. Chavez, Mary G. Gleason. Climate mediates hypoxic stress on fish diversity and nursery function at the land–sea interface. Proceedings of the National Academy of Sciences, 2015; 201505815 DOI: 10.1073/pnas.1505815112

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