Gibson_SciReports_Integrated_Phys

Algal physiological ecology on submarine groundwater discharge (SGD) influenced reefs is likely shaped by intermittent, tidally-driven estuarine conditions that occur with SGD fluxes of fresh-to-brackish groundwater from the subterranean estuary to reef ecosystems. SGD is a common inconspicuous feature worldwide on reefs of basaltic high islands and continental margins. Yet, SGD-driven dynamics of algal physiology are not well understood. To understand how invasive species have physiologically outcompeted native species on many SGD-influenced reefs, physiology in tissue water potential (TWP) regulation, photosynthesis, nitrogen storage, and cellular anatomy were measured across a gradient of SGD-influence, for four Rhodophyte species. Compared with non-SGD conditions, SGD was associated with higher TWP, larger medulla cells with thinner walls, and thinner cortical cell walls for two invasives, <i>Gracilaria salicornia </i>and <i>Acanthophora spicifera</i>, higher photosynthetic rates in <i>G. salicornia</i>, and greater nitrogen concentration for <i>A. spicifera </i>and<i> G. salicornia</i> and increased <i>𝛿</i>¹⁵N ratios for <i>A. spicifera, G. salicornia</i>, and native <i>L. dendroidea</i>. Distinct physiological strategies were measured for the two invasive species across the gradient of SGD-influence, and for <i>L. dendroidea </i>and <i>Gracilaria perplexa</i> offshore. This study illuminates species-specific physiological response, and how introduced opportunistic species may outcompete native species under conditions of SGD.