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MEPS prepress abstract   -  DOI:

Low growth resilience of subarctic rhodoliths (Lithothamnion glaciale) to coastal eutrophication

David Bélanger*, Patrick Gagnon

*Corresponding author:

ABSTRACT: Eutrophication is one of the most important drivers of changes in coastal marine ecosystems worldwide. Given their slow growth, rhodoliths and the biodiverse communities they support are regarded as non-renewable resources threatened by human activity. Consequences of nutrient enrichment on growth and calcification in crustose coralline algae are equivocal, and even more so in cold-water rhodoliths. We paired a 183-d laboratory mesocosm experiment and a 193-d field experiment with Newfoundland rhodoliths (Lithothamnion glaciale) to test the hypothesis that nutrient (nitrate, ammonia, and phosphate) enrichment and biofouling reduce rhodolith growth. Rhodoliths in the laboratory were exposed to one of three nutrient concentrations (ambient, intermediate, or high) and either of two levels of manual cleaning (cleaned or uncleaned) to control biofouling. We exposed rhodoliths in the field to one of two nutrient concentrations (ambient or elevated). Eutrophication in the laboratory did not affect biofouling, however manual cleaning reduced biofouling by ~4 times relative to uncleaned rhodoliths. Rhodoliths grew two times slower at elevated than ambient concentrations, and ~27% more in cleaned than uncleaned rhodoliths at all concentrations. Rhodoliths in the field also grew significantly slower under elevated than ambient phosphate concentrations, but only during the first 6 wk, indicating some capacity for long-term recovery. We conclude that despite some growth resilience to low levels of infrequent increases in nutrient concentrations, subarctic L. glaciale rhodoliths cannot cope with prolonged exposure to modest eutrophication.