Using In-Situ Collections of Reef Algae to Examine Land-Based Sources of Pollution in Kānéohe and Waialua Bays on Óahu, Hawaíi

In healthy coastal waters of Hawaii, nutrient pollution poses a substantial risk to coastal environments. Land-based sources of pollution (LBSP), and in particular, on-site sewage disposal systems (OSDS) can seep into groundwater. These nutrients are eventually introduced to the coastline via submarine groundwater discharge (SGD), and can double ambient nutrient levels. Waialua and Kānéohe Bays have been identified by the State of Hawaíi, Department of Health as regions of concern with high densities of OSDS. Algae incorporate nitrogen (N) from coastal waters into their plant tissues. In-situ algae samples of invasive Acanthophora spicifera, Gracilaria salicornia, (Rhodophyta), as well as Dictyosphaeria versluysii and Ulva lactuca, (Chlorophyta) were opportunistically collected across Waialua and Kānéohe Bays with GPS coordinates recorded. Samples were transported to the laboratory to be processed in preparation of tissue-based assays of ẟ¹⁵N and %N at the UHM SOEST Biogeochemical Isotope Laboratory. Values of ẟ¹⁵N suggest potential LBSP while %N can reveal nutrient loading into coastal ecosystems. For Waialua Bay, Haléiwa beach park had algae that exhibited high ẟ¹⁵N (11.8-12.1‰) and moderate to high %N (1.0-2.7%N), indicating a significant wastewater source. For Kānéohe Bay, comparison across algal data from 2018-2019 revealed comparable values for these parameters, indicating consistent OSDS inputs. However, due to unusual morphologies, some algae such as Dictyosphaeria may access other nutrient pools, diverging from patterns detected with Acanthophora, Gracilaria, and Ulva. Further, preliminary findings also suggest that fine scale mixing and differentiation from small differences in locale collection can affect these parameters. Waialua and Kānéohe Bays have sustained significant nutrient loading in the summer months and further study and continuation of this work will be vital to inform remediation efforts to be carried out by resource managers. Biogeochemical analyses of algal tissues are useful to examine nutrient pollution as they provide relatively inexpensive, but robust assessments of the water quality as opposed to single point water samples which could vary greatly across tides and time.