What Is the Relative Importance of Phytoplankton and Attached Macroalgae and Epiphytes to Food Webs on Offshore Oil Platforms?

The term artificial reef refers to structures constructed or placed in waters for the purpose of enhancing fishery resources and commercial and recreational fishing opportunities. Some artificial reefs are built exclusively for this purpose. Objects constructed for other purposes also act as artificial reefs. In the Gulf of Mexico (Gulf) for example, over 4,000 standing oil and gas platforms function as artificial reefs. Like other artificial reefs, platforms support abundant fouling communities of reef‐dependent biota. However, platforms differ from most artificial reefs in that they rise to and above the sea surface allowing access to the photic zone for benthic primary producers that inhabit platform surfaces. In the Gulf, reef‐associated primary producer communities are dominated by red (e.g., Polysiphonia sp.) and green (e.g., Derbesia sp.) macroalgae and epiphytic microalgae including diatoms and cyanobacteria (Gallaway and Lewbel 1982; Lewbel et al. 1987; Carney 2005). However, the biomass of epibenthic communities on platforms is typically dominated by large, long‐lived, sessile consumers, including barnacles (e.g., Balanus tintinnabulum) and bivalves (e.g., Chama macerophylla and Ostrea spp.), that act as foundation species supporting a high abundance and diversity of small, mobile, reef‐dependent organisms including amphipods, tanaids, harpacticoid copepods, polychaetes, crabs, and small reef‐dwelling fishes (Gallaway and Lewbel 1982; Lewbel et al. 1987).

However, the role of benthic primary producers in food webs on and near artificial reefs remains poorly understood. Benthic macroalgae will be incorporated into the food web as a basal resource if grazed and assimilated by reef‐dwelling consumers, including by suspension feeders that filter macroalgal detritus (Kang et al. 2008; Schaal et al. 2010). Although green algae generally lack chemical or physical defenses and are, therefore, susceptible to grazing (McConnell et al. 1982), some species of red algae are unpalatable (Kain and Norton 1990) and may not contribute to the food web. Benthic microalgae are generally considered highly nutritious for consumers (Miller et al. 1996), and epiphytic microalgae could be directly grazed by mobile consumers from surfaces associated with reefs or consumed by suspension feeders after entrainment into the water column. Alternatively, phytoplankton may be consumed by reef‐dwelling suspension feeders (e.g., barnacles and bivalves: Gallaway and Lewbel 1982) and then redirected to platforms in the form...