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Patterns and filters of eutrophicationendpoints in Elkhorn SloughBrent Hughes, Kerstin Wasson, John HaskinsElkhorn Slough National Estuarine Research Reserve

What is Eutrophication?Source: Associated Press

Conceptual Evolution of CoastalEutrophication•Early models focused on lakes, which have a directresponse to nutrient addition (Volenweider 1976)Direct ResponsesNutrientsPrimary ProductivityOrganic DepositionMicrobial ActivityHypoxiaFrom Cloern (2001)

Conceptual Evolution of CoastalEutrophication•Current models recognize variability in response amongestuaries due to various filters, as well as indirectresponses.NutrientsFiltersTurbidityDepthCurrentsTidesMarshDirect ResponsesPrimary ProductivityOrganic DepositionMicrobial ActivityFrom Cloern (2001)Indirect ResponsesBottom DOCommunityDiversityFood Webs

Study Goals1. Characterize the spatial dynamics ofeutrophication in Elkhorn Slough2. Understand filters of eutrophication, with anemphasis on tidal range.3. Focus on eutrophication within an estuary,instead of between estuaries.

Elkhorn Slough, CA: a highly altered estuary

Eutrophication Filter: Tidal Range

Background information• Elkhorn Slough has high nutrient concentrationsand loading (54.4kg NO 3 /acre/y) (Johnson2008).• Elkhorn Slough has high primary productivityindicating hypereutrophication (860 g/C/m 2 /y)• Tidally restricted areas have decreased benthicdiversity and abundance (Oliver et al. 2009).

Macroalgae: highly abundant at most sitesUlva often nutrient limited, enhanced by agricultural inputs;Algal mats can affect dissolved oxygen and benthic communities

Macroalgal Abundance: related to tidal exchange

Water Column Chlorophyll a: high at all sitesAffects dissolved oxygen and light availability in the water column

Chl a indicates eutrophic conditionsConcentrations not significantly different in full vs. muted sitesEutrophic lines based on Bricker et al. (2003)High EutrophicationModerate Eutrophication

Sediment Anoxia: Affects benthicinvertebrate communities and feeding byother animals.

Sediment Anoxia: related to tidal exchangeA much narrower layer at the top of the mud is well oxygenated atmuted vs. full tidal sites

Hypoxia Events: More frequent in tidallyrestricted areas.

Nutrients do not explain eutrophic patterns(Nitrate/Phosphate)

Nutrients do not explain eutrophic patterns(Nitrate/Phosphate)

Eutrophication End Points

Eutrophication End Points: Explained by Tidal Exchange

Hypoxia Filter: Maximum Tidal Range

Summary•Elkhorn Slough is a highly eutrophic estuary, yet varieswithin the estuary as moderate to hyper eutrophic.•Sites with a decreased tidal range have lower intertidalalgal cover, but more floating algal mats, and higherphytoplankton concentrations.•Sites with a decreased tidal range have a higherfrequency of hypoxia events, and more anoxicsediments.

Conclusions•Tidal range is a strong predictor of degree of eutrophicationbetween sites within the estuary, and thus serves as an important"filter" for expression of eutrophication•Artificial tidal restriction can result in expression of severeeutrophication in highly nutrient loaded estuaries•Even a moderate increase in tidal range behind water controlstructures can lead to substantial improvements in water quality

AcknowledgementsFunding Sources:•NERR Water Quality Program•Community Foundation for Monterey County/PG&E Non-PointSource Pollution Grant (Grant # 20060387)•Elkhorn Slough Foundation•Monterey County Water Resources AgencyField Assistants: Sue Shaw, Ron Eby, Joseph Hatfield, RikkeKvist Preisler, Patty BrownLab Analysis: Sara Tanner (MLML), Moss Landing Marine LabChemical Oceanography Lab, Nick Welschmeyer (MLML),Theresa Hodges (Monterey County Consolidated Chemistry Lab)Other Data Sources: Monterey Bay Aquarium Research Institute/Land Ocean Biogeochemical Observatory (Ken Johnson), A. Gee