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IS MACROALGAL ACCUMULATION A RELIABLE INDICATOR OF INFAUNAL STRESS IN AN OREGON ESTUARY?
Citation:
Young, D., J. Chapman, R. Caldwell, AND J. Lamberson. IS MACROALGAL ACCUMULATION A RELIABLE INDICATOR OF INFAUNAL STRESS IN AN OREGON ESTUARY? Pacific Estuarine Research Society 2015 Annual Meeting, Seattle, WA, March 19 - 21, 2015.
Impact/Purpose:
This research finding relates to EPA’s goal of establishing limits for nutrient loads to estuaries that protect designated uses of these ecosystems. A frequently-used indicator of excess nutrient inputs is the accumulation of macroalgae to levels believed to be harmful to the ecosystem. One measure of such damage is the development of toxic levels of dissolved sulfides in surficial sediment pore water from the accumulation of benthic macroalgae. This study showed that, in one Pacific Northwest coastal estuary, peak season accumulations of macroalgae to levels above 100 gdw m-2 (often accepted as an approximate threshold of damage) occurred at two nearby intertidal sites. At one site there was subsequent development of median dissolved sulfide levels to four times the LC50 value reported for crustaceans and mollusks. However, at a second nearby site, where total infaunal amphipod counts were 17-fold higher, similar accumulations of macroalge were associated with virtually non-detectable sulfide levels. Thus, a major difference in the abundance of this important component of estuarine food webs, associated with a large difference in pore water sulfides, was not associated with the accumulated biomass of benthic macroalgae. This raises questions regarding the reliability of benthic macroalgal accumulation as an indicator of the condition of the benthos in Pacific Northwest estuaries.
Description:
A frequently used indicator of estuarine condition is the accumulation of benthic macroalgae. We compared the biomass of the predominant green macroalgae Ulva, the development of toxic sulfides in surficial sediment pore water, and the abundances of infaunal organisms at high and low sulfide sites in Yaquina estuary, Oregon. At the high-sulfide site the median pore water sulfide concentration in the upper intertidal zone between September and November was 80 µM, three orders of magnitude above that at the low-sulfide site (0.1 µM) and four times the median LC50 reported for crustaceans and mollusks (~20 µM). This suggests that toxic concentrations of dissolved sulfides occurred only at the high-sulfide site, in late summer and early fall. Infaunal amphipod counts at this site were 17-fold lower than at the low-sulfide site, and substantially lower burrow densities of benthic shrimp at the high-sulfide site also were observed. In contrast, average values for macroalgal accumulation at the high- and low-sulfide sites in the peak season agreed within about a factor of two (225 and 120 gdw m-2, respectively), and were not significantly different. These results suggest that macroalgal biomass alone is not a reliable indicator of benthic condition in Pacific Northwest coastal estuaries.