Variability in biomass decay rates and nutrient loss in bloom-forming macroalgal species

This thesis is comprised of one manuscript. A summary of the manuscript follows: Decaying macroalgae release nitrogen and other nutrients into the surrounding marine environment, providing nutrients for future generations of primary producers, as well as fueling a complex web of decomposer microorganisms. There are relatively few studies that examine macroalgal decomposition rates in areas impacted by macroalgal blooms, although fast-growing macroalgal bloom species typically decay more quickly than slow-growing perennial species. We studied whole tissue, organic content, and nutrient decay rates for five macroalgal species in Narragansett Bay, RI in the summer of 2010 using an intertidal litterbag design; four of these species are frequently present in macroalgal blooms in this system. Our results, which we present as logistic decay rates (k), illustrate that the red alga Gracilaria vermiculophylla decomposes most rapidly, followed by ephemeral green algae ( Ulva rigida, Ulva compressa) and the red alga Agardhiella subulata; decay rates for these four species were significantly higher than that for the perennial, non-bloom forming brown alga Fucus vesiculosus. We did not observe refractory pools for any species, though we did not follow F. vesiculosus through the end of its decay process. Decay rates were dependent upon water temperature, with faster decomposition rates occurring during peak temperatures. We speculate that the slow decomposition of F. vesiculosus may be attributable to its relatively high cell wall phenolics, which have been shown in previous studies to retard microorganismal decay. We observed nitrogen spikes during initial decay of F. vesiculosus due to chemical or biological immobilization of allocthonous nitrogen in the algal tissue. Nitrogen and organic material were lost from red species at a faster rate than green or brown species, likely due to faster leaching and/or greater decomposer action. Initial δ15N values varied greatly among species, collection sites, and collection dates and generally did not exhibit predictable changes over the period of decay. Our results are of particular importance in eutrophied systems, where shifting productivity regimes may lead to changes in total nutrient cycling rates, and where changes in stable nitrogen isotopic values have been correlated with nitrogen released from sewage treatment plants. One appendix of decay curves follows the manuscript