Effects of submarine groundwater discharge on bacterial growth efficiency in coastal Hawaiian waters

An unresolved question in microbial oceanography is to what extent do hetero trophic bacteria serve as a carbon (C) and nutrient source for higher trophic levels in food webs. Coastal bacterial growth efficiency (BGE) studies addressing this question have focused largely on river-dominated estuaries, but submarine groundwater discharge (SGD) is also an important freshwater, nutrient, and organic matter source to coastal waters, and its effect on BGE is unknown. We assessed BGE, cell abundance, growth rates, production, respiration, and dissolved organic carbon (DOC) bioavailability in surface waters inside and outside of SGD plumes at 4 sites (2 leeward and 2 windward) on Hawai'i Island. SGD effects on bacterial dynamics were greatest within windward SGD plumes, where discharge rates were highest. SGD effects were minimal within leeward plumes as their values were comparable to those in nearby ocean waters. In windward SGD plumes, BGE and cell abundance were lowest, but bacterial growth rates and DOC bioavailability were highest. Bacterial cell abundance was also inversely related to salinity, suggesting that either SGD diluted marine bacterial cells or that it had lower abundances compared to marine waters. Uncoupling of bacterial production and respiration may explain the inverse patterns observed with BGE and growth rates. Overall, low BGE (8 to 20%) in these coastal waters, especially those with high SGD, suggest that bacteria transfer only a small fraction of their consumed C to the next trophic level, and are possibly a source of CO2 to the atmosphere.