Joel S. Steward

Setting seagrass depth, coverage, and light targets for the Indian River Lagoon system, Florida

Seagrass protection and restoration in Florida’s Indian River Lagoon system (IRLS) is a mutual goal of state and federal programs. These programs require, the establishment of management targets indicative of seagrass recovery and health. We used three metrics related to seagrass distribution: areal coverage, depth limit, and light requirement. In order to account for the IRLS’s spatial heterogeneity and temporal variability, we developed coverage and depth limit targets for each of its 19 segments. Our method consisted of two steps: mapping the union of seagrass coverages from all availabe mapping years (1943, 1986, 1989, 1992, 1994, 1996, and 1999) to delineate wherever seagrass had been mapped and determining the distribution of depth limits based on 5,615 depth measurements collected on or very near the deep-edge boundary of the union coverage. The frequency distribution of depth limits derived from the union coverage, along with the median (50th percentile) and maximum (95th percentile) depth limits, serve as the seagrass depth targets for each segment. The median and maximum depth targets for the IRLS vary among segments from 0.8 to 1.8 and 1.2 to 2.8 m, respectively.Halodule wrightii is typically the dominant seagrass species at the deep-edge of IRLS grass beds. We set light requirement targets by using a 10-yr record of light data (1990–1999) and the union coverage depth limit distributions from the most temporally stable seagrass segments. The average annual light requirement, based on the medians of the depth limit distributions, is 33 ± 17% of the subsurface light. The minimum annual light requirement, based on of the 95th percentile of the depth distributions, is 20 ± 14%; the minimum growing season light requirement (March to mid September) is essentially the same (20 ± 13%). Variation in depth limits and light requirements, is probably due to factors other than light that influence the depth limit of seagrasses (e.g., competition, physical disturbance). The methods used in this study are robust when applied to large or long-term data sets and can be applied to other estuaries where grass beds are routinely monitored and mapped.

Setting load limits for nutrients and suspended solids based upon seagrass depth-limit targets

Total nitrogen (TN), total phosphorus (TP), and total suspended solids (TSS) loadings [log (kg ha−1 yr−1)] were regressed against seagrass depth limits (percent of depth-limit targets) to back-predict the load limits or allocations (kg ha−1 yr−1 or kg yr−1) necessary to meet targeted seagrass depth limits in the Indian River and Banana River (IRBR) lagoons, Florida. Because the load allocations can be applied as total maximum daily loads (TMDL) for the IRBR (U.S. Environmental Protection Agency mandate), the method and results are developed and presented toward that end. The regression analyses indicate that the range of surface-discharge load limits (nonpoint + point source), per watershed area, required to achieve the desired depth limits for seagrass in the IRBR are approximately 2.4–3.2 kg ha−1 yr−1 TN, 0.41–0.64 kg ha−1 yr−1 TP, and 48–64 kg ha−1 yr−1 TSS. This simple regression method may have application to other shallow estuarine lagoons or bays where seagrass growth is limited by light and water transparency, water transparency is strongly affected by watershed pollutant loadings, water residence times are sufficiently long to allow seagrass coverage to respond to and covary with total load inputs, and multiyear monitoring has yielded sufficient variability in both pollutant loadings and seagrass coverages to develop a statistically meaningful relationship.

Cutting through complexity with Vollenweider's razor

R. A. Vollenweider applied the principle of parsimony to develop models of lake trophic state that cut through the considerable complexities of lake ecosystems. Based on the mass balance of phosphorus, these models reduced the complex aquatic biogeochemistry of phosphorus to a single term that accounted for the loss of phosphorus to lake sediments. Simple as they were, the models were highly predictive as was first convincingly demonstrated by the recovery trajectory of Lake Washington. Subsequently, Volledweiders approach was demonstrated to be sufficiently robust to span the latitudes from temperate to tropical regions and to span the freshwater to saltwater continuum. Vollenweider was a pioneer in predictive ecology and among the first to demonstrate that simple, robust, and understandable models can provide the basis for sound ecosystem management.