Armitage NvsP 2005

We examined the spatial extent of nitrogen (N) and phosphorus (P) limitation of each of the major benthic
primary producer groups in Florida Bay (seagrass, epiphytes, macroalgae, and benthic microalgae) and characterized the
shifts in primary producer community composition following nutrient enrichment. We established 24 permanent 0.25-
m2 study plots at each of six sites across Florida Bay and added N and P to the sediments in a factorial design for 18
mo. Tissue nutrient content of the turtlegrass Thalassia testudinum revealed a spatial pattern in P limitation, from severe
limitation in the eastern bay (N:P . 96:1), moderate limitation in two intermediate sites (approximately 63:1), and
balanced with N availability in the western bay (approximately 31:1). P addition increased T. testudinum cover by 50–75%
and short-shoot productivity by up to 100%, but only at the severely P-limited sites. At sites with an ambient N:P ratio
suggesting moderate P limitation, few seagrass responses to nutrients occurred. Where ambient T. testudinum tissue N:P
ratios indicated N and P availability was balanced, seagrass was not affected by nutrient addition but was strongly
influenced by disturbance (currents, erosion). Macroalgal and epiphytic and benthic microalgal biomass were variable
between sites and treatments. In general, there was no algal overgrowth of the seagrass in enriched conditions, possibly
due to the strength of seasonal influences on algal biomass or regulation by grazers. N addition had little effect on any
benthic primary producers throughout the bay. The Florida Bay benthic primary producer community was P limited, but
P-induced alterations of community structure were not uniform among primary producers or across Florida Bay and did
not always agree with expected patterns of nutrient limitation based on stoichiometric predictions from field assays of
T. testudinum tissue N:P ratios.