Abstract
Seagrass communities, dominated by Thalassia testudinum, form the principal benthic ecosystem within Florida Bay, Florida USA. The bay has had several large-scale seagrass die-offs in recent decades associated with drought and hypersaline conditions. In addition, three category-5 hurricanes passed in close proximity to the bay during the fall of 2005. This study investigated temporal and spatial trends in macrophyte abundance and water quality from 2006 to 2013 at 15 permanent transect sites, which were co-located with long-term water quality stations. Relationships, by year and by transect location (basin), between antecedent water quality (mean, minimum and maximum for a 6-month period) and benthic macrophyte communities were examined using multivariate analyses. Total phosphorus, salinity, pH, turbidity, dissolved inorganic nitrogen (DIN), DIN to phosphate ratio (DIN:), chlorophyll a, and dissolved oxygen correlated with temporal and spatial variations in the macrophyte communities. Temporal analysis (MDS and LINKTREE) indicated that the fall 2005 hurricanes affected both water quality and macrophyte communities for approximately a 2-year period. Spatial analysis revealed that five basins, which subsequently exhibited a major seagrass die-off during summer 2015, significantly differed from the other ten basins in macrophyte community structure and water quality more than 2 years before this die-off event. High total phosphorus, high pH, low DIN, and low DIN:, in combination with deep sediments and high seagrass cover were characteristic of sites that subsequently exhibited severe die-off. Our results indicate basins with more mixed seagrass communities and higher macroalgae abundance are less susceptible to die-off, which is consistent with the management goals of promoting more heterogeneous benthic macrophyte communities.
Highlights
Seagrasses form the dominant benthic ecosystem in Florida Bay; they are the dominant physical structure in the bay (Hall et al, 2007; Herbert et al, 2011)
In the CLUSTER plot and multi-dimensional scaling (MDS) ordination for the environmental data (Figures 3A,B), 2006 and 2007 were separated from the later years, indicating water quality in the 2 years following the passage of the three category-5 hurricanes was most dissimilar to the subsequent 6 years
The water quality variables best correlated with patterns in the biological communities were: maximum total phosphorus, mean dissolved inorganic nitrogen (DIN):PO−4 3, minimum chlorophyll a, minimum salinity, and maximum turbidity
Summary
Seagrasses form the dominant benthic ecosystem in Florida Bay; they are the dominant physical structure in the bay (Hall et al, 2007; Herbert et al, 2011). The most common seagrass species in Florida Bay are Thalassia testudinum, Halodule wrightii, and Syringodium filiforme (Hall et al, 1999, 2007). Historic changes in water usage in the Everglades have caused a 60% decline in freshwater inflow into the Florida Bay area (Herbert et al, 2011). These historic changes have caused an increase in salinity in Florida Bay, with increased occurrences of hypersalinity and reduced salinity variability (Herbert et al, 2011). The reduction in freshwater runoff and resulting high salinity has led to “marinization” and increased homogeneity in the vegetation in the system, causing T. testudinum to become more widespread and abundant as other seagrasses have declined in abundance (Zieman, 1982; Fourqurean and Robblee, 1999)
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