Mangrove structure, litter and macroalgal productivity in a northern‐most forest of Florida

Abstract

Rhizophora mangle L. dominated 10 overwash islands within Tampa Bay forming the northernmost mangrove forests on the west coast of Florida. The mean number of trees and basal area were 5040 trees ha-1 and 20.5 m2 ha-1, respectively. Basal areas ranged from 1.1 (Avicennia germinans (L.) Stern), to 2.6 m2 ha-1 (Laguncularia racemosa Gaertner), to 16.8 (R. mangle). Cockroach Bay mangroves are small (5.8–7.0 m tall) versus coastal forests of south Florida and the Caribbean. Total litter production for a 12 month period was maximal in September for fringing (7.4 g d wt m-2 d-1) and interior (8.7 g d wt m-2 d-1) areas with the two zones not being significantly different. Average litter fall (3.1 g d wt m-2 d-1) was similar to more tropical Caribbean mangals. Leaves accounted for 68 of the litter and reproductive material for 6, being similar to tropical riverine and overwash marine angiosperm communities on the Mexican coast. Although mangrove forests in Tampa Bay are small in stature and experience cold damage and occasional frosts, their litter fall is similar in biomass to that of more tropical mangals. However, their reproductive output is low based on litter fall versus more tropical mangals suggesting that the northern extension is less than optimum. Macroalgal diversity was low (10 species) as compared to Caribbean mangals. Macroalgal turf (0.8 g C m-2 d-1) and epiphyte communities of A.germinans pneumatophores (2.7 g C m-2 d-1) have productivity levels that are equal to or greater than those of Caribbean mangals.