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Marine Ecology Progress Series

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MEPS 519:29-45 (2015)  -  DOI: https://doi.org/10.3354/meps11099

Endemic grazers control benthic microalgal growth in a eutrophic tropical brackish ecosystem

T. S. Sakihara1,4,*, B. D. Dudley2, R. A. MacKenzie3, J. P. Beets1,5

1Tropical Conservation Biology and Environmental Science Program, University of Hawaii at Hilo, 200 West Kawili Street, Hilo, HI 96720, USA
2Department of Biology, University of Hawaii at Hilo, 200 West Kawili Street, Hilo, HI 96720, USA
3Institute of Pacific Islands Forestry, Pacific Southwest Research Station, USDA Forest Service, 60 Nowelo Street, Hilo, HI 96720, USA
4Present address: Division of Aquatic Resources, Department of Land and Natural Resources, State of Hawaii,
1160 Kamehameha Avenue, Hilo, HI 96720, USA
5Present address: Department of Marine Science, University of Hawaii at Hilo, 200 West Kawili Street, Hilo, HI 96720, USA
*Corresponding author:

ABSTRACT: Anthropogenic changes to nutrient supply, numbers and behavior of grazers and interactions of these factors are known to change epilithon composition and biomass. In brackish waters, these changes occur across wide-ranging abiotic conditions (e.g. nutrient concentrations and salinity), which may alter their relative impacts on microphytobenthic communities. Such mediating processes are poorly understood, particularly in tropical brackish ecosystems. We examined the separate and interactive effects of nutrient additions and grazer exclusion on epilithon composition and biomass in eutrophic Hawaiian anchialine pools between contrasting levels of salinity and concomitant nutrient-rich groundwater influence (2.3 to 22.0 ppt; nitrite + nitrate, 6.3 to 102.6 µM; soluble reactive phosphorus, <0.5 to 5.18 µM). Across these conditions, we found no significant effects of nitrogen (N) and phosphorus (P) enrichment treatments on chlorophyll a, biomass (ash-free dry mass) or autotrophic index (i.e. autotrophy) of epilithon communities, and nutrient addition did not alter the effects of grazing pressure. However, autotrophy and epilithon biomass were lower in low salinity, high groundwater influenced pools, and both were strongly reduced by grazing in all pools. Furthermore, effects from grazing pressure on relative autotrophy were more pronounced in low salinity, high groundwater influenced pools. Our results suggest that (1) grazing is the primary driver of epilithon composition and biomass across these nutrient-enriched systems, (2) microphytobenthic communities in these systems are not N or P limited irrespective of co-varying salinity and background nutrients and (3) selective feeding by endemic grazers mitigates potential bottom-up forces from salinity or increased nutrient-enriched groundwater influence on autotrophy in epilithon communities.


KEY WORDS: Anchialine pool · Top-down control · Bottom-up control · Epilithon · Grazing · Nutrient enrichment · Halocaridina rubra


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Cite this article as: Sakihara TS, Dudley BD, MacKenzie RA, Beets JP (2015) Endemic grazers control benthic microalgal growth in a eutrophic tropical brackish ecosystem. Mar Ecol Prog Ser 519:29-45. https://doi.org/10.3354/meps11099

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