Relative effects of nutrient enrichment and grazing on epiphyte-macrophyte (Zostera marina L.) dynamics

Abstract

The independent and interactive effects of nutrient concentration and epiphyte grazers on epiphyte biomass and macrophyte growth and production were examined in Zostera marina L. (eelgrass) microcosms. Experiments were conducted during early summer, late summer, fall, and spring in a greenhouse on the York River estuary of Chesapeake Bay. Nutrient treatments consisted of ambient or enriched (3× ambient) concentrations of inorganic nitrogen (ammonium nitrate) and phosphate. Grazer treatments consisted of the presence or absence of field densities of isopods, amphipods, and gastropods. epiphyte biomass increased with both grazer removal and nutrient enrichment during summer and spring experiments. The effect of grazers was stronger than that of nutrients. There was little epiphyte response to treatment during the fall, a result possibly of high ambient nutrient concentrations and low grazing pressure. Under low grazer densities of early summer, macrophyte production (g m d) was reduced by grazer removal and nutrient enrichment independently. Under high grazer densities of late summer, macrophyte production was reduced by enrichment only with grazers absent. During spring and fall there were no macrophyte responses to treatment. The relative influence of epiphytes on macrophyte production may have been related to seasonally changing water temperature and macrophyte requirements for light and inorganic carbon.

Ecosystem recovery after climatic extremes enhanced by genotypic diversity

“…In this respect, reported effects of eelgrass genotypic diversity on faunal abundance should be regarded as conservative estimates. Herbivorous gastropods and juvenile bivalves constitute important components of the food web in seagrass ecosystems as they crop epiphytic and planktonic algae, respectively, and may enhance the persistence of seagrass beds even in the face of eutrophication (50,51). Larger, mobile consumers such as fish and crabs also use seagrass habitat as feeding and breeding habitat (12).…”

Eutrophication in shallow coastal bays and lagoons: the role of plants in the coastal filter

“…Numerous experimental and observational studies from coastal bays indicate the capacity of a variety of benthic grazers including filter-feeding bivalves, ascidians, sponges, polychaetes, amphipods, gastropods, and small herbivorous fish to buffer the effects of nutrient enrichment (e.g. Alpine & Cloern 1992, Neckles et al 1993, Williams & Ruckelshaus 1993, Lin et al 1996, Heck et al 2000, Hillebrand et al 2000, Lotze & Worm 2002, Heck & Valentine 2007). For example, dense populations of filter-feeding bivalves limit the accumulation of phyto-8 Fig.…”

“…Mesograzers (e.g. amphipods and gastropods) can control the abundance of algal epiphytes on seagrasses and hard substrates even under moderate nutrient loading (Neckles et al 1993, Williams & Ruckelshaus 1993, Hillebrand et al 2000, Lotze & Worm 2002. Likewise, macroalgal responses to eutrophication, and the shading impact of macroalgal blooms on seagrasses, may be mediated by grazing .…”

Strong bottom‐up and top‐down control of early life stages of macroalgae

“…Their relative strength, however, may vary under different environmental conditions. Almost all of our scant knowledge on the interactions of consumer and resource control of marine macroalgal communities comes from studies on adult life stages (GeertzHansen et al 1993;Neckles et al 1993;Hauxwell et al 1998). However, in contrast to pelagic primary producers, macroalgae have complex life cycles where different life stages of the same species are part of the planktonic, microbenthic, and macrobenthic communities (Santelices 1990).…”