A qualitative and quantitative assessment of seaweed decomposition in the Strait of Georgia

Appropriate sampling and experimental programs resulted in a qualitative and quantitative assessment of seaweed litter biomasses, decomposition rates and concomitant changes in nitrogen content; detritus biomass and decomposition rates; and faunal distribution patterns for the significant species within a successional seaweed community in the Strait of Georgia, British Columbia, Canada. A simulation model incorporating suitable data obtained from these sampling and experimental programs facilitated prediction of detritus formation rates, biomass, nitrogen content and the seasonal availability of detritus as a food resource for fauna. Soluble matter release rates from decomposing seaweed litter and its nitrogen content were also determined. Of the ca 43 taxa identified within the seaweed litter collections, Fucus distichvs L. (41%), Irldaea cordata (Turner) Bory (26%), Nereocystis 1uetkeana (Mertens) Postels and Ruprecht (27%), and Laminaria (4%) (L. saccharina (L.) Lamouroux and L. groenlandica Rosenvinge) accounted for more than 97% of total litter deposition. The mean peak summer biomass of all litter was ca 5 g ash-free dry weight (AFDW)/m² with this figure approaching zero during January and February. Litter distribution was patchy and there was sufficient evidence to conclude that most litter was retained, and underwent decomposition, in the immediate vicinity of its place of deposition. Litter decomposition experiments performed on the 10 most significant contributors to seaweed community structure indicated that decomposition of seaweed litter occurs rapidly compared to vascular plant litter. The time required for seaweed litter to disappear from 2 mm mesh litter bags ranged from six days, for the lamina of Nereocystis luetkeana, to ca 70 days, for Fucus distichus. Some similarity in decomposition rates was observed amongst species displaying taxonomic and/or morphologic affinities. Assessment of nitrogen content of decomposing seaweed litter revealed that nine of the 10 species assayed lost nitrogen less rapidly than total litter biomass. As determined by assaying microbial consumption of particulate material, the time required for detritus (particle size < 1 mm, dry) to fully decompose was short. Of the 10 species tested, Iridaea cordata detritus decomposed most rapidly at a rate of 5.7% per day while rates for Gigartina papillata (C. Agardh) J. Agardh, Laminaria groenlandica, Laminaria saccharina and Nereocystis luetkeana ranged from 2-4% per day. Data for the remaining species were less conclusive although all decomposed at rates less than one percent per day. Variation in specific decomposition rates was shown to be correlated with the structural composition of the detritus. Those species with a relatively small percentage of crude fibre as a component of their particulate fraction decomposed more rapidly than those species with a higher percentage of crude fibre. For the two most rapidly decomposing species, Iridaea cordata and Nereocystis luetkeana, a trend toward a more rapid decomposition rate as mean particle size decreased was evident. Natural detritus (particle size < 2 mm, wet) biomass accumulation within the study site peaked at ca 1.4 g AFDW/m² during the latter half of August 19 76. This value represents 1-5% of the quantity of detritus predicted to have been formed from seaweed litter alone and a lesser percentage of the total quantity of seaweed detritus formed. Exportation out of the seaweed zone is believed to be responsible for this discrepancy. The predicted rates of detritus formation and soluble matter release from decomposing seaweed litter peaked at ca 0.6 and 0.5 g AFDW/m²per day, respectively, in early September 1976 from a low near zero in February. In total, ca 56% of litter biomass formed detritus, the remainder being released as soluble matter. The mean nitrogen contents of the detritus formed and the soluble matter released were 2.48 ± 0.03% and 1.36 ± 0.03% of their dry weights, respectively. The annual contribution of seaweed litter biomass via detritus and soluble matter to local coastal waters is estimated to be in the range of 70-85 g C/m². Detritus formed from seaweed litter was determined to have a C:N ratio of 10-13:1, rendering it suitably nutritious for utilization by fauna as a food resource, however it could not be shown conclusively that the coincidence, en masse, of specific fauna and maximum detritus availability was a response to the availability of detritus as a food resource. The possibility of such a correlation is discussed with reference to two species of caprellids, Caprella alaskana Mayer and Metacaprella anomala Mayer, and the benthic gastropod Lacuna marmorata Dall.

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