Zonation pattern and spatial arrangement of a Geukensia granosissima population in a mixed mangrove forest of Tampa Bay.

摘要

Here I provide the first report on Geukensia granosissima patterns of abundance along a tidal gradient within a mixed mangrove stand located in Tampa Bay, Florida, USA. Specifically, I examined 1) the relationship between G. granosissima size and density with mangrove root type (e.g. prop root, pneumatophore), and of density within the intertidal zones; and 2) the possible role of predation in shaping the lower zonation patterns displayed. Transect surveys located along the lower and upper population limit boundaries were conducted every two months over a ten-month period. Variables measured include size distribution, density of mussels, above ground mangrove prop and pneumatophore roots. To evaluate potential predator influence on mussel distribution, predator exclusion experiments were conducted in March and June 2016, using mock pneumatophore platforms at both high and low tidal elevations. Surveys indicated that over all dates mean mussel densities and percent cover were higher along the lower limit tidal elevation [mean (+/- SD) = 1280.3 +/- 665.9 m-2 and 20.6 +/- 3.78% respectively], versus that in the higher limit tidal elevation [102.4 +/- 50.7 m-2 and 0.52 +/- 0.17%]. Survivorship of mussels in the predator exclusion platforms placed at higher position within lower edge of the mussel tidal distribution was approximately 100% on both experimental dates. During March 2016, mussel survivorship was lower when predators had access to mussels with the greatest loss of mussels in the lower (26% survivorship) vs. higher (66.5%) tidal elevations after 48h. Similarly, when predators had access to mussels in June experiments, after just 24 h mussel loss was greater at lower (1% survivorship) vs. higher (80% survivorship) tidal elevations, possibly reflecting differences in predator densities, identities, and/or functional responses. Overall trends in the patterns of distribution of this population of G. granosissima suggest that the lower tidal boundary of the mussel is shaped by predation but that predation plays a lesser role in the high tidal areas.