I.M., Fluorescent dissolved organic matter as a multivariate biogeochemical tracer of submarine groundwater discharge in coral reef ecosystems, Marine Chemistry (2015), AbstractIn Hawaiʻi and other Pacific high islands submarine groundwater discharge (SGD) can be a significant and continuous source of solutes to nearshore reefs and may play a key role in the structure and function of benthic coral and algal communities.Identifying SGD sources and linking them to reef biogeochemistry is technically challenging. Here we analyzed spectra of fluorescent dissolved organic matter (fDOM) in coral reefs in the context of a suite of biogeochemical parameters along gradients of SGD to characterize fDOM composition and evaluate the utility of fDOM signatures in tracking groundwater dispersal and transformation. We spatially mapped water column chemistry in Maunalua Bay, O'ahu, Hawaiʻi by collecting 24 water samples in grids at each of two ~0.15 km 2 regions during both high and low tides over a two-day period. We observed clear horizontal gradients in the majority of 15 measured parameters, including inorganic and organic solutes and organic particles that tracked concentrations of conservative SGD tracers (radon, salinity and silicate). Multivariate scanning excitation-emission fluorometry successfully differentiated two distinct groundwater sources and delineated regions of SGD dispersion in each reef from the surrounding water column samples without detectable groundwater. Groundwater was consistently depleted in DOC and enriched in nutrients; although the two SGD sources varied widely in fDOM quantity and fluorophore proportions, indices of humification were consistently elevated in SGD at both sites. Our results provide a robust spectral characterization of fDOM in SGD-influenced coral reefs and indicate the potential for this rapid and cost-effective measurement technique to be useful in tracking SGD dispersal in nearshore ecosystems.

It is critical to evaluate the in situ effects of multiple stressors on coastal community dynamics, especially those communities harboring high diversity such as coral reefs, in order to understand the resilience of these ecosystems, prepare coastal management for future scenarios, and aid in prioritizing restoration efforts. In this in situ study, at 2 sites with gradients of submarine groundwater discharge (SGD), a suite of physical parameters (wave exposure index, wind exposure index, and depth) and an all-encompassing SGD chemical parameter (average nitrate + nitrite daily load) were measured along spatially cohesive and temporally relevant scales and used to model macroalgal growth, biomass, and diversity in Maunalua Bay, Hawai'i. We showed that (1) speciesspecific macroalgal biomass is significantly related to SGD and one of the 2 exposure indices (i.e. wind exposure or wave exposure), (2) SGD and wave exposure play key roles in species-specific growth rates, and (3) SGD supports low diversity and increased biomass of species that can tolerate the biogeochemistry associated with SGD. Our work suggests that SGD and local hydrodynamics predict local variation in macroalgal growth, biomass, and diversity in tropical reefs.

Some introduced species compete directly with native species for resources and their spread can alter communities, while others do not proliferate and remain benign. This study compares community structure and diversity in adjacent areas dominated by the introduced alga Avrainvillea sp. or native algal species on a hard substrate reef. The biomass and species composition of 15 paired plots (30 in total, plot type based on dominance of Avrainvillea sp. or native species) were quantified. Plots dominated by Avrainvillea sp. had a significantly different assemblage of species characterized by lower algal diversity, mostly Dictyota spp. and Laurencia sp., and a higher abundance and diversity of invertebrates, such as small arthropods, polychaetes, and brittlestars. These results suggest that as Avrainvillea sp. becomes more abundant on hard substrate reefs, it will engineer a different community composed of algal epiphytes and an invertebrate assemblage more typically associated with algae in soft sediments.

Spatially broad and long-term monitoring studies are lacking in tropical intertidal systems yet are necessary to test predictions regarding community assembly. To fill this gap, we examined spatial and decadal temporal patterns in benthic community structure at rocky intertidal sites along the main islands of Hawai'i. Quantitative community surveys done in 2017 across nine sites and five islands showed that organismal composition differed by site, substrate type, and island. Secondly, we leveraged an earlier dataset collected using the same methods and analyzed intertidal communities at five sites on three Hawaiian islands for temporal changes in organismal abundance and composition from 2006 and 2007 vs. 2016 and 2017. Overall community structure differed significantly across years and decades. Most decadal differences were site specific, such as the fivefold increase in turf algae at one site. Crustose coralline algae and Turbinaria ornata increased significantly across five sites; both are physically resilient algae and similar increases in their abundances have been observed in tropical systems worldwide. This increase in physically resilient macroalgal species is potentially caused by global drivers, such as rising temperatures and changing land uses. In conclusion, there is evidence that both local and regional factors contribute to structuring tropical intertidal communities.