Abstract

Stable isotope studies in seagrass beds have shown that food webs in these essential habitats are based mainly on algal rather than on seagrass carbon. Epiphytic algae have received the lion’s share of the attention but the trophic role of the understory sediment microalgae may be just as important. Furthermore, nutrient enrichment of coastal waters may result in replacement of seagrass beds by unvegetated sediments where all benthic production based on chlorophyll a would then be that of the sediment microalgae.

We therefore propose to determine the effects of nutrient enrichment on the trophic importance of sediment microalgae associated with seagrass beds. In Year 1, different spatial positions within and adjacent to beds of Halodule wrightii will be enriched with 14N-enriched fertilizer to simulate eutrophication and serve as a tracer of N (and organic matter) flow within the system.

The following responses of the sediment microalgae to nitrogen enrichment will be quantified: primary production rates (mg C m-2 h-1), HPLC photopigments (proxies for biomass), and the structure (species present and their relative abundances) of the sediment diatom assemblages. ð15N values of primary producers and consumers will be determined to trace the uptake of the added nitrogen and its movement through the food web.

The relative importance of epiphytic versus sediment microalgae will be quantified by examining the assemblage structure of diatoms present in the guts of selected consumers. P-I curves will be determined for the sediment microalgae within and adjacent to seagrass beds to develop a map of spatial and monthly changes in their primary production rates following a GPS survey of irradiance levels, as a preliminary step in placing their function in a landscape perspective.

In Year 2, the effects of grazers (density and size will be manipulated) and nutrient enrichment and their interactions on sediment microalgae utilization as a food source will be determined by a series of experiments involving the placement of exclosures and enclosures within H. wrightii beds.

At the end of each experiment photopigments and diatom assemblage structure will be determined, as well as ð15N values of surviving animals. The proposed research addresses a major priority of the MASGC Strategic Plan for 2000-2005, namely that of understanding impacts of eutrophication (N enrichment) on living marine resources (sediment microalgae and the animal species that consume them).

Results of this research will be published in peer-reviewed journals, presented at national meetings, and communicated to appropriate personnel of the Gulf Islands National Seashore for management purposes. Furthermore, the public will benefit through a better understanding of how unseen microscopic organisms (i.e. the sediment microalgae) have a major impact on coastal processes such as the production of the seafood they love so much. This will be accomplished through interactions of the PI with outreach specialists identified in the proposal narrative.

Objectives

1. To determine the utilization of sediment microalgae within and adjacent to seagrass beds by different size classes of grazing animals
2. To determine the interactions between grazer size and density and nutrient enrichment on utilization of sediment microalgae as a food source within seagrass beds. 

Methodology

Objective 1 will be accomplished by placing exclosures within and adjacent to Halodule wrightiibeds to exclude only macrograzers (larger mesh size) or both micro- and macrograzers (smaller mesh size). At the end of the experiment the biomass (HPLC photopigments) and the structure of the sediment diatom assemblages will be analyzed and compared to control plots. Objective 2 will be realized by placing enclosures with H. wrightii beds only and either no animals, micrograzers only, or macrograzers only will be added. For a given grazer density, half of the enclosures will be subjected to enrichment with a 14N-enriched fertilizer as in Year 1. At the end of the experiment photopigments and diatom assemblage structure will be determined, as well as δ 15N values of surviving animals. Appropriate statistical analyses will be run for both the exclosure and enclosure experiment series.

Rationale

Stable isotope studies in seagrass beds have shown that food webs in these important fisheries habitats are based mainly on algal rather than on seagrass carbon. Epiphytic algae have received the lion’=s share of the attention but the trophic role of the sediment microalgae is unknown and may be just as important. Furthermore, as stated in the MASGC Strategic Plan, it is imperative to understand the impacts of eutrophication (nitrogen enrichment) on living marine resources (benthic microalgae and the animals that consume them) in coastal 
ecosystems and habitats. In contrast to the parsimonious distribution of seagrass beds and their epiphytic algae, the sediment microalgae occupy vast areas of the benthic landscape on a global basis; thus, their potential as an organic matter source for coastal and nearshore fisheries may be enormous and this hypothesis urgently needs testing.