Effects of Nitrogen Availability and Herbivory on Eelgrass (Zostera Marina) and Epiphytes
Although the growth of eelgrass (Zostera marina) is controlled by resources as well as higher order interactions with epiphytes and their herbivores, these constraints rarely are considered together. The ability to utilize both water column and sediment nutrient sources in a complex habitat may provide eelgrass with a partial release from nutrient competition with epiphytes that have more efficient uptake kinetics and can reduce eelgrass growth, particularly in eutrophic habitats. We investigated the relative effects of dissolved inorganic nitrogen in the water column vs. the sediments, and herbivory by the common isopod Idotea resecata, on eelgrass growth and epiphyte biomass in an intertidal eelgrass bed in Padilla Bay, Washington. In the field, we fertilized the sediments and/or the water column with ammonium and measured eelgrass growth and epiphyte biomass. We also monitored epiphyte biomass and water column nutrient concentrations and censused isopod densities. Laboratory experiments focused on the effects of I. resecata, fertilization of the water column and sediments, and depletion of sediment nutrients on eelgrass growth and epiphyte biomass. Most simply, we hypothesized that epiphytes would respond positively to increased water column nutrients as eelgrass would to increased sediment nutrients, and that herbivory on epiphytes could mitigate deleterious effects of epiphytes on eelgrass. We demonstrated that eelgrass growth is affected both by sediment nitrogen resources and the higher order effects of epiphytes and their control by Idotea resecata. During our field experiments, growth of eelgrass leaves tended to increase in response to sediment fertilization; this trend was significant in April 1988 but not in August 1987. At both times, leaf growth rates demonstrated a saturation-type response to sediment ammonium concentrations >100 @mmol/L, providing further support for nitrogen limitation of eelgrass growth over much of the range in ambient concentrations (30-137 @mmol/L) in the sediment porewaters. Together, sediment ammonium concentrations and epiphyte biomass explained a significant portion (71%) of the variance in eelgrass leaf growth in August 1987. Consideration of sediment nitrogen, epiphytes, or herbivores alone is unlikely to yield a predictable understanding of the control of eelgrass primary productivity in nature, particularly given the complexity of the eelgrass habitat with respect to its dual nutrient sources. For example, epiphyte biomass was predicted by our laboratory experiments and other data to be nitrogen limited in Padilla Bay, yet it was not correlated with ambient nitrogen concentrations nor did it increase with fertilization of the water column. These results can be reconciled by considering herbivory by Idotea resecata. In the laboratory, the isopod reduced epiphyte biomass by one-third and in its absence, epiphyte biomass increased with increasing nitrogen concentrations in the water column and negatively affected eelgrass growth.
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