Veronica L. Gibson1, Celia M. Smith1
1Department of Botany, School of Life Sciences, University of Hawaiʻi at Mānoa
While polluted submarine groundwater discharge (SGD) has been linked to algal bloom conditions on nearshore reefs, neither macroalgal physiological response to SGD nor benthic community ecology under SGD conditions are well understood. A cryptic yet common feature on basaltic coastlines, SGD delivers tidally-driven semidiurnal pulses of fresh, nutrient rich basal groundwater to Hawaii’s nearshore ecosystems. SGD influenced nearshore ecosystems are characterized by daily oscillations in salinity, temperature, and nutrient availability. Natural gradients of SGD influence on nearshore reefs may have provided strong selective pressures on macroalgal species, leading to niche partitioning along this gradient. To examine the influence of SGD on nearshore macroalgal physiology and ecology a multi-factor analysis is employed. Field characterization across a gradient of polluted SGD influence measures physiological responses in photosynthesis and tissue water potential regulation by four macroalgal species to SGD conditions at Waiʻalae Beach. Comparisons are made across two species pairs, closely related invasive Gracilaria salicornia and cryptogenic Hydropuntia perplexa, and invasive Acanthophora spicifera and closely related native Laurencia dendroidea. Measurably different physiological strategies are exhibited in these species. Additionally, benthic analyses describe community diversity and structure across the SGD gradient. Benthic analyses reveal the highest macroalgal coverage, low diversity, and invasive species dominance nearest to SGD seeps, while higher diversity, low macroalgal coverage, and more native species occurred at the offshore, non-SGD influenced site. We discuss implications for the role of SGD and algal physiological response to SGD for watershed management and ecosystem restoration.