Kū Hou Kuapā: Increase of Water Exchange Rates and Changes in Microbial Source Tracking Markers Resulting from Restoration Regimes at He‘eia Fishpond.

Moehlenkamp, Paula

Kū Hou Kuapā: Increase of Water Exchange Rates and Changes in Microbial Source Tracking Markers Resulting from Restoration Regimes at He‘eia Fishpond.

Files

2018-08-ms-moehlenkamp.pdf (40.41 MB)

Date

2018-08

Authors

Moehlenkamp, Paula

Department

Oceanography

Abstract

Anthropogenic activities have changed island ecosystems throughout history. Hawaiʻi’s natural environment has been dramatically altered by land use change, urbanization, pollution, and the introduction of invasive species causing a demise of traditional Hawaiian fishponds across the state over the last century. Heʻeia fishpond is currently being restored and provides- embedded between land and sea- a unique opportunity to examine how historical land use change has altered the functions of coastal habitats and how restoration can help to maintain and improve the integrity of coastal ecosystems in the face of rapid global change. He‘eia fishpond is an example of a traditional Hawaiian aquaculture system at the terminus of He‘eia ahupuaʻa on the windward site of Oʻahu, Hawaiʻi. It is a natural embayment that is enclosed by a constructed wall (kuapā) with sluice gates (mākāhā) facilitating water exchange crucial for fish survival. This study examines how major restoration regimes, as the removal of invasive mangroves, and the reconstruction of a 50 m section of the kuapā known as “Ocean Break”, impacted water exchange rates, residence times, salinity distribution, as well as abundance of microbial source tracking markers. Our study revealed that Heʻeia fishpond’s physical environment is largely tidally driven during baseline (non-storm) conditions with wind forcing and river flux being secondary drivers. Postrestoration, two (OM1/Mākāhā Nui, Kahoʻokele (former OB)) of six mākāhā accounted for over 80% of relative flux together, making the northeastern region of the fishpond the dominant flow pathway of water into and out of the fishpond. The repair of Ocean Break increased water exchange rates ~5% during spring tide and ~16% during neap tide and similarly decreased minimum water residence time in the fishpond from 38 hours to 32 hours and maximum residence time from 102 hours to 64 hours. Salinity distribution displayed a spatial gradient across the fishpond with higher salinities on the ocean side of the fishpond and lower salinities towards the fresh water dominated site. Comparison of pre- vs. post-restoration salinity revealed significantly lower average salinities post-restoration, an indication for increased fresh water flux due to mangrove removal around the northern fishpond periphery. Spatial distribution of microbial source tracking markers was inversely correlated with salinity. Despite decreased residence times, average abundance of Enterococcus and Bacteroidales did not significantly change after restoration efforts. As these microbes are introduced through freshwater from terrigenous runoff, the increase in fresh water flushing post-restoration presents a mechanism increasing overall abundance, hence counteracting the positive impact increased exchange rates may have on water quality. However, average abundance of Fusobacteria, a biomarker specific to fecal contamination from cattle egrets living at the fishpond, decreased significantly after restoration. The source of bird microbial contamination lies in the fishpond and is less dependent on terrigenous freshwater input suggesting that increased flushing affected bird biomarker abundance. Taken together microbial source tracking is a promising avenue to pursue further in understanding how restoration and changes in circulation relate to microbiological water quality assessments. Repairing the wall restored the fishpond to its traditional nature: A loko kuapā - a seashore fishpond with an artificial stone wall enclosing the system during all tidal states and sluice gates facilitating rigorous water exchange in particular in the eastern portion of the fishpond. To avoid events with mass fish mortality in the future, we recommend moving fish pens strategically to the eastern region of the fishpond (close to Kahoʻokele and OM1), which exhibit the highest flushing rates with favorable conditions for fish to thrive. This study clearly demonstrates the positive impact restoration regimes have had on water flushing and water quality parameters encouraging the prospect of revitalizing this culturally and economically significant site for sustainable aquaculture in the future.

Keywords

Heʻeia Fishpond, community restoration, conservation ecology

URI

http://hdl.handle.net/10125/62654

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M.S. - Oceanography

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