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Application of a Eutrophic Condition Index to Benthic Macroalgal Accumulation in Pacific Northwest Estuaries
Citation:
Young, D., Pat Clinton, H. Lee, C. Brown, D. Specht, AND R. Caldwell. Application of a Eutrophic Condition Index to Benthic Macroalgal Accumulation in Pacific Northwest Estuaries. Presented at Pacific Estuaries Research Federation Annual Meeting, Newport, OR, April 03 - 05, 2014.
Impact/Purpose:
An assessment framework for macroalgal eutrophication, recently developed for the European Union Water Framework Directive and used to demonstrate substantial macroalgal eutrophication of southern California estuaries, has limited relevance to Pacific Northwest (PNW) coastal estuaries. Its application to benthic macroalgal percent cover and biomass data from a five-year study in Yaquina estuary, Oregon yielded similar low Eutrophic Condition Index (ECI) values for the two major study sites where similar summer algal accumulation, but radically different pore water sulphide concentrations, were measured. Average Fall levels of dissolved sulphides at one site averaged ten times the LC-50 values reported for benthic invertebrates, but were undetectable at the other site. This absence of sensitivity to differences in a major toxic condition represents an important limitation to this approach for characterizing macroalgal impacts. Also, this index does not distinguish between cultural eutrophication, common in other regions, and natural eutrophication such as occurs in PNW coastal estuaries. Further, the Poor to Bad summer macroalgal eutrophic condition indicated by the ECI for Yaquina estuary contrasted with the lack of a relationship observed between macroalgal and native eelgrass abundance, even at relatively high macroalgal levels (>700 g dw m-2). In addition, Yaquina estuary is a major contributor to the recreational shellfish industry in the region. Results of application of the ECI to macroalgal accumulation values from a probabilistic 13-estuary study in the PNW were consistent with published reports that benthic macroalgal growth is supported mainly by upwelled nutrients rather than those released from the watershed. However, without the extensive and substantive research efforts of marine scientists in the PNW, application of this assessment framework alone would have yielded highly misleading results regarding the occurrence of benthic macroalgae in PNW coastal estuaries.
Description:
Studies of benthic macroalgal accumulation in coastal estuaries of the Pacific Northwest, USA, were conducted over a 12-year period, including aerial mapping and ground surveys. The results were applied to an assessment framework for eutrophication developed by the European Union and recently used to evaluate macroalgal accumulation in the Southern California Bight. A detailed five-year, fixed-transect survey in Yaquina estuary, Oregon revealed large temporal and spatial variations in average accumulation and corresponding eutrophic condition index (ECI) values during the summer-fall period of highest percent cover and biomass. Two sites with similar average macroalgal accumulation and ECI values had very different levels of sediment pore water sulfides, known to be highly toxic to benthic organisms. Thus, other factors (e.g., water flow, sediment porosity, bioturbation) not included in the ECI may be important in determining the impact of high macroalgal accumulation on the benthic environment. However, application of this ECI to macroalgal results from probabilistic field surveys of 13 PNW coastal estuaries conducted between 2004 and 2009 generally yielded ratings between “good” and “high” indicating little or no eutrophication in these systems, in contrast to results reported for many of the southern California estuaries. Further, aerial monitoring of summer macroalgal extent in Yaquina estuary between 1997 and 2009 revealed no indication of a systematic increase in abundance of this benthic macrophyte.