Anthropogenic nutrient loading is well recognized as a stressor to coastal ecosystem health. However, resource managers are often focused on addressing point source or surface water discharge, whereas the impact of submarine groundwater discharge (SGD) as a nutrient vector is often unappreciated. This study examines connections between land use and nutrient loading through comparison of four watersheds and embayments spanning a gradient of human use impact on Tutuila, a high tropical oceanic island in American Samoa. In each study location, coastal radon-222 measurements, dissolved nutrient concentrations, and nitrogen isotope values (δ¹⁵N) in water and in situ macroalgal tissue were used to explore SGD and baseflow derived nutrient impacts, and to determine probable nutrient sources. In addition to sampling in situ macroalgae, pre-treated macroalgal specimens were deployed throughout each embayment to uptake ambient nutrients and provide a standardized assessment of differences between locations. Results show SGD-derived nutrient flux was more significant than baseflow nutrient flux in all watersheds, and δ¹⁵N values in water and algae suggested wastewater or manure are likely sources of elevated nutrient levels. While nutrient loading correlated well with expected anthropogenic impact, other factors such as differences in hydrogeology, distribution of development, and wastewater infrastructure also likely play a role in the visibility of impacts in each watershed.

Groundwater dependent ecosystems (GDEs) are increasingly recognized as important conservation targets with linked ecological and social value. However, the social uses and values of GDEs have received relatively little research attention in the peer-reviewed literature, precluding their greater inclusion in policy and management decisions. To help fill this gap, we provide a case study from Kona, Hawaiʻi, where multiple types of GDEs are abundant, to illustrate the diversity of social uses and values of GDEs. To explore these uses and values, we combined a literature review, archival analysis, and key-informant interviews with resource managers and lineal descendants connected to three prominent GDEs: Indigenous aquaculture systems, anchialine pools, and nearshore ecosystems. Interviews focused on current and historical uses and values of GDEs, contemporary management challenges and strategies, and desired visions for the future. Interviewees expressed a range of uses and values associated with GDEs, which we categorized using a Hawaiʻi-based cultural ecosystem service framework focused on social connections, physical and mental health, spirituality, and knowledge. Importantly, results suggest that the historical value of these systems directly informs current social value, and that restoration efforts are largely carried out through biocultural approaches, which emphasize the mutually reinforcing restoration of ecology and culture. We found that interviewees seek to restore ecosystem functions, cultural practice, and connection to place, and in some cases, local food production. Achieving these goals requires addressing multiple and interacting threats to these systems including invasive species, land-based sources of pollution, groundwater pumping, and climate change. Importantly, effective and equitable restoration also rests on recognition and amplification of Indigenous rights, knowledge, practice, and governance. These results provide important lessons for land and water management and policy in Hawaiʻi as well as other islands and coastal areas where GDEs have important linked social and ecological value.

Regulation of tissue water potential is a key mechanism in macroalgal osmotic responses to changing external osmotic conditions, which are common in tidally influenced estuarine and intertidal systems. Nevertheless, significant knowledge gaps exist in our understanding of osmotic responses in macroalgae because few methods measure osmotic potential within macroalgal tissues. Leaf psychrometers have furthered understanding of osmotic potentials in terrestrial plant water relations, yet these have not been developed to measure the range of highly negative potential values found in marine macroalgae. To address these gaps, we present an effective, updated version of the Chardakov method to measure tissue water potential in macroalgae. Here, we present a case study examining macroalgal response in tissue water potential by two morphologically and evolutionarily distinct species, Ulva lactuca (Chlorophyta) and Hypnea musciformis (Rhodophyta) to four paired salinity and nutrient treatments at two temperatures. These treatments simulate a gradient from full coastal ocean conditions to brackish submarine groundwater discharge, an ecosystem type found on basaltic shorelines. Both algae demonstrated plasticity in osmotic response to submarine groundwater discharge with significant positive correlations between tissue water potential and proportion of submarine groundwater discharge in the treatment. These results are the first to describe macroalgal response in tissue water potential, a first step to understanding algal physiological ecology in such complex coastal environments. This revised Chardakov method is a valuable tool to better understand species-specific osmotic responses to ecologically relevant conditions, and can augment the study of other tidal systems and ontogenetic stages.

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, Gracilaria salicornia and Acanthophora spicifera, higher photosynthetic rates in G. salicornia, greater nitrogen concentration for A. spicifera and G. salicornia, and increased \(\delta\)¹⁵N ratios for A. spicifera, G. salicornia, and native Laurencia dendroidea. Distinct physiological strategies were measured for the two invasive species across the gradient of SGD-influence, and for L. dendroidea and Gracilaria perplexa offshore. This study illuminates species-specific physiological response, and how introduced opportunistic species may outcompete native species under conditions of SGD.