Seasonal occurrence patterns of seagrass should influence resource assessment and management decisions: A case study in the Indian River Lagoon and Loxahatchee River Estuary, Florida

Abstract

Seagrasses provide and maintain critical ecosystem functions and support rich biodiversity, yet they are highly susceptible to anthropogenic impacts. From 2002 to 2016 seagrasses were surveyed monthly or bimonthly for presence in the Indian River Lagoon (IRL) and Loxahatchee River Estuary (LRE) along the east coast of Florida. These data were used to assess seasonal seagrass presence patterns (i.e., annual maxima) and their relevance to management initiatives. Three species were examined (Halodule wrightii, Syringodium filiforme, and Halophila johnsonii) at 37 sites spanning 200 km of the IRL and 5.3 km of the LRE. Seagrass annual maxima, defined as bimonthly maximum annual percent presence of visible above-ground blades, exhibited clear seasonal trends: 61% of H. wrightii annual maxima (across years) occurred between May and August, 71% of S. filiforme annual maxima occurred between May and October, and 65% of H. johnsonii annual maxima occurred between March and June. Surveys intended to document and protect the spatial extent of seagrasses should, therefore, be scheduled during periods of expected annual maxima to capture the full extent of seagrass presence. These findings stress that any conservation-based seagrass monitoring program should conduct sampling efforts that account for the considerable seagrass seasonality. At a minimum, the survey period for H. johnsonii, a federally listed “threatened” species, should be revised to include March through June as an addendum to the current state and federal guidelines of June through September to provide better protection for this species. The results of this study suggest seagrass surveys for dredge and fill projects, dock permits, or other activities potentially damaging to seagrasses, should be conducted during periods when the annual maxima are most likely. Doing so will afford a more appropriate level of protection for these valuable ecosystem components.