Anthropogenic nutrient loading and subsequent eutrophication is one of the greatest threats facing our coastal ecosystems. I have been studying estuaries along the west coast of the United States where there is great variation in nutrient loading on both local and regional scales. For the latter, I use human land use data to determine human footprints in watersheds that drain into the coastal environment. Below is an example of potential threats from land to the coastal environment at two estuaries with greatly different nutrient levels:
Focusing in on Elkhorn Slough (the watershed to the right), I have been investigating many of the ecological impacts of intense nutrient enrichment from the surrounding agricultural landscape. Specifically, I have been determining how physical and chemical drivers influence eutrophication (defined as the increase in the rate of supply of organic matter to an ecosystem). The expression of eutrophication is highly influenced by physical processes (i.e. tidal forces) which can mask the harmful effects of anthropogenic nutrient loading. Below is an example of how eutrophication is expressed in a single estuary:
Hughes, B.B., J. Haskins , K. Wasson, and E. Watson. 2011. Identifying factors that influence expression of eutrophication in a central California estuary. Marine Ecology Progress Series 439:31-43.
Wasson, K., R. Jeppesen, C. Endris, D. Perry, A. Woolfolk, K. Beheshti, M. Rodriguez, R. Eby, E. Watson, F. Rahman, J. Haskins, B.B. Hughes. 2017. Eutrophication decreases salt marsh resilience through proliferation of algal mats. Biological Conservation. 212:1-11.
Coastal Hypoxia and Habitat Alteration
Often times human threats don’t come alone, they often come in pairs or more, making it challenging to assess their relative or synergistic effects. Part of my research investigates how multiple stressors affect the functioning and diversity of ecosystems. My research has focused primarily on how eutrophication interacts with other stressors, such as habitat alteration and ocean acidification. More recently, I have been advancing this research to look at how eutrophication and the interaction with tidal restriction influences hypoxia over decadal scales in Elkhorn Slough. Using the fish assemblage as a model group I am determining how variation in hypoxia influences the diversity and abundance of fish, the climatic forces that affect hypoxia and subsequently fish, and the influence of nursery conditions in Elkhorn Slough on nearshore adult populations in Monterey Bay. For the latter I am using flatfish, primarily English sole (Parophrys vetulus), to test for the effects of hypoxia in its primary nursery grounds (Elkhorn Slough) on the offshore adult population and fishery.
Hughes, B.B., M. Fountain, A. Carlisle, M. Levey, M. Gleason. 2012. The impacts of nutrient loading and environmental conditions on the fish assemblage and available nursery habitat in Elkhorn Slough. The Nature Conservancy.
Hughes, B.B., M.D. Levey, M.C. Fountain, A.B. Carlisle, F.P. Chavez, M.G, Gleason. 2015. Climate mediates threats to fish diversity and nursery function at the land-sea interface. Proceedings of the National Academy of Sciences USA 112:8025-8030.
Jeppesen, R., *M. Rodriguez, *J. Rinde, J. Haskins, B.B. Hughes, L. Mehner, K. Wasson. 2016. Hypoxia increases fish mortality and reduces oyster growth in a highly eutrophic estuary. In press. Estuaries and Coasts.
Eutrophication and Ocean Acidification
Climate change has brought about profound changes in ocean chemistry through the process of ocean acidification. This along with increased upwelling along the California current and increased eutrophication can ultimately draw down pH in coastal systems that can fundamentally change their functioning. Along with Kristy Kroeker (UCSC), we are looking into the future by creating future acidification scenarios and combining it with an additional stressor of anthropogenic nutrient loading to look at species interactions and productivity of eelgrass Zostera marina. We are using mesocosm experiments that manipulate pH, nutrients, and consumers to investigate tipping points related to the interaction between shade causing algal epiphytes and eelgrass.
Mesocosm array at the University of California Santa Cruz’s Long Marine Lab testing for the effects of nutrient loading (eutrophication) and Ocean Acidification on seagrass communities. (Left) Recent seagrass transplants getting acclimated to their new acidic home. (Middle) The mesocosm (barrel) set up at the Tank Farm. (Right) The brain where all of the magic happens.
Hughes, B.B., S.C. Lummis, S.C. Anderson, K.J. Kroeker. 2017. Unexpected resilience of a seagrass system exposed to global stressors. In press. Global Change Biology.