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

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Eutrophication is one of the most important drivers of change 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 with a 193 d field experiment on Newfoundland (Canada) 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 1 of 3 nutrient concentrations (ambient, intermediate, or high) and either of 2 levels of manual cleaning (cleaned or uncleaned) to control biofouling. We exposed rhodoliths in the field to 1 of 2 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 2 times slower at elevated than ambient nutrient 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 and infrequent increases in nutrient concentrations, subarctic L. glaciale rhodoliths cannot cope with prolonged exposure to modest eutrophication.