Ecohydrology of Macroalgae and Submarine Groundwater Discharge on Hawaiian Reefs

Abstract

Interactions among ecological and hydrological processes have emerged as important areas of research as anthropogenic degradation of watersheds and climate change impact groundwater dependent ecosystems (GDE). Coastal reefs are examples of GDE in many tropical high islands; these reefs are subject to significant fresh, nutrient-rich-groundwater which creates estuarine conditions on nearshore reefs. To better understand benthic community dynamics on submarine groundwater discharge (SGD) influenced reefs, benthic cover and biological drivers of community composition at three SGD-influenced sites were analyzed. Generally, nearshore SGD-influenced regions have higher macroalgal coverage than marine offshore regions. Two of the three study sites were dominated in the nearshore SGD-influenced region by two invasive species, while the third site was dominated by native turf and macroalgae. Benthic community analyses reveal that tolerance of hypo-osmotic conditions could strongly influence the dominance of invasive species in low salinity, higher nutrient SGD-influenced regions. The physiological trait that allows plants to respond to hypo-osmotic conditions such as SGD conditions is ability to adjust tissue water potential (TWP). To investigate SGD responses, I updated a method to measure TWP in macroalgae and applied this method to invasive macroalgae that span the SGD-gradient and more narrowly, oceanic distributed native species at Waiʻalae ʻIki. Additionally measured responses included photosynthesis, cellular anatomy, and nitrogen content of four species of Rhodophyta to the SGD-gradient at Waiʻalae ʻIki. Two invasive Rhodophyta exhibited strategies of invasive biology with measurable responses consistent with success under SGD conditions; native species responses remained elusive. Further, in controlled growth experiments, TWP responses tracked simulated SGD-conditions by one Chlorophyte and one Rhodophyte, not found at Waiʻalae ʻIki. Understanding the relationships between SGD and macroalgal physiological ecology is pivotal to protecting native macroalgal diversity and ecosystem function, and to mitigating the effects of anthropogenic watershed degradation and climate change on SGD-influenced nearshore reefs.