Surface and groundwater influence on spatial distribution of geochemistry and water quality in Maunalua Bay

Paula Moehlenkamp1, Kim A Falinski2, Craig E Nelson3, Margaret Anne McManus4, Doug Harper5, Nyssa Silbiger6, Henrietta Dulai7, Christina Mae Comfort8, Gordon Walker8 and Megan Donahue9, (1)University of Hawai‘i at Mānoa, Oceanography, Honolulu, United States, (2)The Nature Conservancy, Honolulu, HI, United States, (3)University of Hawaii at Manoa, HI, United States, (4)University of Hawaii at Manoa, Department of Oceanography, Honolulu, HI, United States, (5)Malama Maunalua, United States, (6)California State University Northridge, Biology, Northridge, CA, United States, (7)University of Hawaiʻi at Mānoa, Earth Sciences, Honolulu, HI, United States, (8)University of Hawaii at Manoa, Oceanography, Honolulu, HI, United States, (9)University of Hawai‘i at Manoa, Hawai‘i Institute of Marine Biology, Kane‘ohe, HI, United States
Maunalua Bay in southeast Oʻahu, Hawaiʻi is a fringing coral reef ecosystem surrounded by urbanized watersheds that have been significantly changed by stream channelization, harbor dredging, land reclamation, and on-site sewage disposal. Over time, these changes in land-use, as well as overfishing, have led to the degradation of coastal coral reefs affecting populations of marine organisms and impacting the ecological, economic, and cultural value of the Bay.

The present study aims to better understand the linkage between anthropogenic activities, surface water and groundwater quality, and coastal biogeochemistry in Maunalua Bay and to contextualize prior studies at focal locations in the Bay with comprehensive synoptic sampling. Over four days in August 2019, we collected ~180 nearshore and offshore water samples at low tide across Maunalua Bay. Water samples were analyzed for nutrients, chlorophyll, total suspended solids, carbonate system parameters, fluorescent dissolved organic matter (fDOM), cytometric counts, and microbial community composition and contextualized with in-situ measurements of physical water quality parameters (temperature, salinity, oxygen, turbidity, pH). Simultaneous measurements by a current meter deployed in the Bay provided further context for water movement. The high spatial resolution of sampling sites allowed us to map water column chemistry across Maunalua Bay and the mixing of submarine groundwater discharge (SGD) and terrestrial run-off. Together this comprehensive data set allows us to characterize the sources and distribution of distinct geochemical signatures of human activities in the Bay. Furthermore, the data supports strategic prioritization and management by state agencies and community-based organizations involved in resource management and restoration of this biologically and culturally significant site.