Competition for waterborne food resources among tropical shallow-water sponges

We examined filtration by sponge assemblages in the shallow waters (~2 m
depth) of Florida Bay (Florida, USA), where water residence times are
often high and filtration by dense communities of sponges were
hypothesized to deplete the water column of food, primarily picoplankton
and dissolved organic matter (DOM). We transplanted three sponge species
into replicate locations that differed by an order of magnitude in natural
sponge community biomass. Sponge transplants were clones, enabling us to
control for sponge genotype effects across all sites. The growth of sponge
clones was recorded seasonally for 18 – 30 months. Growth of transplants
placed in areas devoid of sponges was 10 times greater than in areas with
dense sponge communities, and three times greater than in areas with
average sponge biomass. Sponge mortality was similar regardless of
background sponge density. Measures of picoplankton, DOM, and PO4
concentration confirmed an inverse relationship with sponge community
biomass, whereas nitrogen concentrations in seawater were highest where
sponge species replete with nitrogen-fixing symbiotic microbial
communities were most abundant. This is striking evidence that filtration
of waterborne resources by sponges in shallow, coastal environments can
deplete those resources sufficiently to cause exploitative competition
that limits sponge growth, with cascading implications for tropical
hardbottom environments where sponges dominate the animal biomass.