Algae Eaters Help Clean Up Our Messes
Nuisance algal blooms are a growing problem for Hawaii’s coral reef ecosystems. Damaging algal blooms on O’ahu and Maui have been linked to man-made nutrient inputs delivered to coral reefs via submarine groundwater discharge. Fish, urchin, and turtle grazing on coral reefs can provide numerous ecosystem benefits, including controlling algal growth (by grazing) and potentially reducing associated coral reef decline. As coastal land-use and nutrient inputs increase over time, grazing processes may naturally control nuisance algal growth and ecosystem decline. A new study is investigating the response of algae to nutrient inputs, and the ability of grazers to control algal growth. As coastal development activities and fishing intensity steadily increase in Hawaii, the National Park Service needs scientific understanding to develop appropriate management strategies.
Objectives: Experimental sampling stations were established at Kaloko-Honokōhau National Historical Park (KAHO) on Hawai’i Island, and Kalaupapa National Historical Park (KALA) on Moloka’i Island. From May 2011 to October 2013, experiments were deployed to investigate and compare, 1) benthic (seafloor) nutrient inputs to the reef habitat, 2) algal response (growth) to nutrient inputs, 3) grazer community composition and grazing intensity, and 4) grazer control (consumption) of algal growth.
Benthic water samples have been collected and analyzed for dissolved nitrate + nitrite, ammonium, and phosphate. Preliminary results have documented significantly higher nitrate + nitrite concentrations at KAHO, and significantly higher ammonium concentrations at KALA. Nutrient source determination using stable isotope analysis (15N) is currently underway.
Algal turf growth was measured experimentally using grazer exclusion cage treatments (see photo below). Algal turf growth when grazers were excluded was significantly higher at KALA and was correlated with higher ammonium (NH4) concentration. At KAHO, grazing by fish and urchins significantly reduced algal turf growth. Significant relationships between algal turf growth and nutrient concentrations were not observed at KAHO.
Grazer communities were very different in the two parks: Herbivorous fish biomass (a weight calculation that accounts for fish size) was significantly higher at KALA, while observed urchin densities were very low. Alternatively, KAHO hosted very high densities of large, roving urchins that vigorously graze algae throughout the day and night, including banded urchins (wana, Echinothrix calamaris), red pencil urchins (ha ‘uke ‘uke ‘ula ‘ula, Heterocentrotus mammillatus), and collector urchins (hawa’e maoli, Tripneustes gratilla). Herbivorous fish density (the number of individuals within a survey area) was actually also significantly higher at KAHO, however individual fish tended to be smaller, resulting in a lower calculated biomass relative to KALA. Grazing by the Endangered Green Sea Turtle (Honu, Chelonia mydas) occurred frequently in shallow zones and occasionally along the deeper reef at both parks.
Preliminary results suggest that both nutrient inputs (bottom-up controls) and grazing (top-down controls) influence algal growth at the two Hawaii parks. Final results will provide data on resource conditions in both parks and provide information for management of these important marine resources.
For more information, please contact: Jim Beets, Univ. of Hawai’i at Hilo, (808)932-0137, Beets@Hawaii.edu — Sallie Beavers, Kaloko-Honokōhau NHP,(808)329-6881 x1220, Sallie_Beavers@nps.gov — or Eric Brown, Kalaupapa NHP, (808)567-6802, x1502, Eric_Brown@nps.gov