Regulation of benthic microalgal and meiofaunal productivity and standing stock in a salt marsh ecosystem: the relative importance of resources and predation
Foreman, Kenneth H.
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To examine whether abundance and productivity within salt marsh benthos is controlled by resources (so -called "bottom-up" regulation) or by predacious epifauna (so-called "top-down" regulation), food supply was enhanced by fertilizing experimental plots in the field. Predation pressure was reduced by excluding fish, shrimp and crabs from fertilized and control plots using cages. Benthic microalgal and meiofaunal abundance was monitored for 17 months to assess how controls on abundance varied seasonally. Microalgae and meiofauna in the creek bottoms bloomed each spring. Peak abundances occurred in May-June. Fertilization clearly stimulated microalgae during the bloom. Accumulation of benthic chlorophyll ~ over winter - spring could be predicted from dissolved inorganic nitrogen concentrations in sediment porewater using a modified Monod equation. Mean annual chlorophyll concentrations were 60% higher (198 vs. 123 mg m-2 ) in fertilized than control treatments . Benthic microalgae constitute the principal high quality food for deposit feeding in fauna, and the effects of enrichment were propagated up the food chain. Copepod and nematode populations, in particular, increased in response to fertilization. Meiofauna and microalgae crashed during summer, and differences between fertilized and control treatments disappeared. By September-October no differences could be detected between treatments. Within cages, however, benthic chlorophyll and meiofauna abundance remained high throughout the summer, and differences between fertilized and control treatments persisted. In late summer , 10 weeks after caging began, meiofaunal densities inside fertilized cages were lOX those outside cages. Densities inside control cages were only 4X those outside cages. Phaeopigments accumulated within cages, due to grazing of benthic algae by infauna. Infauna were unable to depress chlorophyll standing stocks as severely as epifauna. Gross production and community respiration, measured as whole sediment 02 flux, was higher in cores from caged than uncaged treatments . Not all meiofaunal taxa responded positively to enrichment. Ostracod abundance was reduced by enrichment, and Foraminifera differed little among treatments. Complex, taxon specific, interactions be tween caging and fertilization treatments occurred, eg. some groups which responded positively to fertilization outside cages were unaffected by enrichment inside cages. Spartina litter of different initial quality (%N and phenolic content) was harvested from fertilized and control plots, placed in litterbags and its decay was monitored. Changes in the taxonomic composition of meiofauna colonizing the litterbags were related to both plot effects and differences in litter chemistry.
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