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- Knowledge of nutrient pathways and their resulting ecological interactions can alleviate numerous environmental problems associated with nutrient increases in both natural and managed systems. Although not unique, coastal systems are particularly prone to complex ecological interactions resulting from nutrient inputs from both the land and sea. Nutrient inputs to coastal systems often spur ulvoid macroalgal blooms, with negative consequences for seagrasses, primarily through shading, as well as through changes in local biogeochemistry. We conducted complementary field and mesocosm experiments in an upwelling-influenced estuary, where marine-derived nutrients dominate, to understand the direct and indirect effects of nutrients on the macroalgal–eelgrass (Zostera marina L.) interaction. In the field experiment, we found weak evidence that nutrients and/or macroalgal treatments had a negative effect on eelgrass. However, in the mesocosm experiment, we found that a combination of nutrient and macroalgal treatments led to strongly negative eelgrass responses, primarily via indirect effects associated with macroalgal additions. Together, increased total light attenuation and decreased sediment oxygen levels were associated with larger effects on eelgrass than shading alone, which was evaluated using mimic algae treatments that did not alter sediment redox potential. Nutrient addition in the mesocosms directly affected seagrass density, biomass, and morphology, but not as strongly as macroalgae. We hypothesize that the contrary results from these parallel experiments are a consequence of differences in the hydrodynamics between field and mesocosm settings. We suggest that the high rates of water movement and tidal submersion of our intertidal field experiments alleviated the light reduction and negative biogeochemical changes in the sediment associated with macroalgal canopies, as well as the nutrient effects observed in the mesocosm experiments. Furthermore, adaptation of ulvoids and eelgrass to high, but variable, background nutrient concentrations in upwelling-influenced estuaries may partly explain the venue-specific results reported here. In order to manage critical seagrass habitats, nutrient criteria and macroalgal indicators must consider variability in marine-based nutrient delivery and local physical conditions among estuaries.
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- Hessing-Lewis, M. L., Hacker, S. D., Menge, B. A., McConville, S. O., & Henderson, J. (2015). Are large macroalgal blooms necessarily bad? Nutrient impacts on seagrass in upwelling-influenced estuaries. Ecological Applications, 25 (5), 1330-1347. doi:10.1890/14-0548.1
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- Thanks to the field and laboratory support associated with this research. At Oregon State University (OSU): O. Moulton, P. Zarnetske, L. Wisehart, D. Eerkes-Medrano, W. Rice-Narusch, H. Bailey, J. Schaefers, M. Rogers, L. McCoy, B. Dumbauld, and T. Gouhier. Research in Coos Bay was funded from an award to M. Hessing-Lewis and S. D. Hacker from the National Estuarine Reserve System (NERR), Estuarine Reserves Division, NOAA. At South Slough NERR, A. Helms, A. DeMarzo, and S. Rumrill facilitated data acquisition and processing. A Mamie Markham Graduate Scholarship provided funds for the mesocosm experiment at Hatfield Marine Science Center (OSU). Science Center staff greatly facilitated construction and maintenance of this experiment. Additional support for this research came from ZoRF grants (Department of Integrative Biology [DIB], formerly Zoology, at OSU), and discretionary funds to S. D. Hacker (from DIB) and B. A. Menge (from the Wayne and Gladys Valley Foundation). M. Hessing-Lewis has been funded by the Tula Foundation through the Hakai Network at Simon Fraser University, and by the Hakai Institute during part of the writing of the manuscript.
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