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Intertidal Eelgrass Response to Benthic Macroalgal Accumulation in a Pacific Northwest Estuary
Citation:
Young, D. AND Pat Clinton. Intertidal Eelgrass Response to Benthic Macroalgal Accumulation in a Pacific Northwest Estuary. Coastal and Estuarine Research Federation, Portland, OR, November 08 - 12, 2015.
Impact/Purpose:
The accumulation of “excessive” levels (e.g., biomass, percent cover) of benthic macroalgae is an important candidate indicator of unacceptably high nutrient loadings to, or concentrations in, estuarine waters. However, arriving at a defensible definition of “excessive accumulation” for a given system is challenging. Here we studied the accumulation (biomass and percent cover) of benthic macroalgae (Ulva spp.) in the intertidal zone of the Pacific Northwest’s Yaquina Estuary, and compared our results to those for the intertidal native eelgrass (Zostera marina) in the ecosystem. This seagrass often is considered to be a critically important component of a healthy estuary in the Northern Hemisphere. Our ground surveys over five peak growth seasons at one 1000 m2 intertidal site showed no indication that macroalgal summertime average biomass levels (ranging from 54 to 430 dry grams per square meter) had any effect on the corresponding above-ground biomass levels of eelgrass measured in the same area the following year. Also, annual aerial photo surveys of standard monitoring areas in the lower estuary between 1997 and 2009 showed no relationship between the areal extents (percent cover) of the macroalgae and eelgrass, and no significant temporal trend for either aquatic plant species. These results provide no indication that the levels of macroalgal accumulation measured in this study had any effect on the abundance (biomass or cover) of native eelgrass in Yaquina Estuary.
Description:
High accumulations of benthic macroalgae from excessive nutrient inputs to estuaries is commonly cited as a major cause of seagrass decline. Two measures of macroalgal abundance, biomass and percent cover, have been used in an assessment framework for estuarine condition proposed by the European Union’s Water Framework Directive. Here we examined the relationship between biomass and cover of the macroalga Ulva, and above-ground biomass and cover of the native eelgrass Zostera marina, in Yaquina Estuary on the central Oregon coast, USA. Random samples of macroalgae and eelgrass were collected during exposed conditions from 1000 m2 intertidal monitoring zones over five peak growth (~ summer) seasons. A comparison of macroalgal and eelgrass cover from annual aerial photo surveys of the entire lower estuary over a twelve year period also was made. An eight fold variation of average macroalgal peak season biomass (54-430 gdw m-2) during the five year ground survey was obtained at one site. However, there was no relationship between the average macroalgal biomass there during one summer season, and the average above-ground biomass of eelgrass at that site the following summer. Also, from the photo surveys conducted between 1997 and 2009 there was no relationship between the aerial extent of eelgrass and macroalgae in the lower estuary, and no temporal change in the percent cover measurement of either taxon. Therefore, our results suggest that benthic macroalgae do not have clear negative effects on native eelgrass in Yaquina Estuary.