The general objective of seagrass monitoring in the Florida Keys National Marine Sanctuary (FKNMS) is to measure the status and trends of seagrass communities to evaluate progress toward protecting and restoring the living marine resources of the Sanctuary. The scope and depth of this monitoring effort are without precedent or peer for seagrass ecosystems throughout the world. Specific objectives are: 1) To provide data needed to make unbiased, statistically rigorous statements about the status and temporal trends of seagrass communities in the Sanctuary as a whole and within defined strata; 2) To help define reference conditions in order to develop resource-based water quality standards; and 3) To provide a framework for testing hypothesized pollutant fate/effect relationships through process-oriented research and monitoring. In order to meet these objectives, we have developed these goals for the project:

To reach these goals, four kinds of data are being collected in seagrass beds in the FKNMS:

These data are being collected at three different types of sites within the FKNMS:

For the first time in the history of this project, we report significant changes in seagrass communities that are consistent with model predictions of nutrient-induced changes of these systems. There may be reasons for these observations that are unrelated to man’s activities in the region, but the spatial pattern of changes and the agreement of the changes with models of the system suggest that there is regional-scale change in nutrient availability that is causing changes in seagrass beds over a wide portion of the FKNMS.

In general, nutrient addition to aquatic environments shifts the competitive balance to faster-growing primary producers. The consequences of this generality in seagrass-dominated environments is that seagrasses are the dominant primary producers in oligotrophic conditions. As nutrient availability increases, there is an increase in the importance of macroalgae, both free-living and epiphytic, with a concomitant decrease in seagrasses because of competition for light. Macroalgae lose out to even faster-growing microalgae as nutrient availability continues to increase: first, epiphytic microalgae replace epiphytic macroalgae on seagrasses; then planktonic microalgae bloom and deprive all benthic plants of light under the most eutrophic conditions. The south Florida case is more complicated than the general case described above because there are 6 common seagrass species in south Florida, and these species have different nutrient and light requirements, hence they have differing responses to eutrophication. Large expanses of the shallow marine environments in south Florida are so oligotrophic that biomass and growth of even the slowest-growing local seagrass species, Thalassia testudinum, are nutrient-limited; at this very oligotrophic end of the spectrum, increases in nutrient availability actually cause increases in seagrass biomass and growth rate. As nutrient availability increases beyond what is required by a dense stand of T. testudinum, there are other seagrass species that will out-compete it (Figure 2). The relative importance of the various primary producers, then, can be used to assess the trophic state of the community.

Each species in the species dominance-eutrophication gradient model (Figure 2) can potentially dominate over a range of nutrient availability and the model predicts a change in species dominance as nutrient availability changes. These changes are not instantaneous, however. Field evidence suggests that species replacements may take place on a time scale of a decade or more. It is desirable that we be able to predict the tendency of the system to undergo these changes in species dominance before they occur, so that management actions can be taken. Tissue nutrient concentrations can be monitored to assess the relative availability of nutrients to the plants. For phytoplankton communities, this idea is captured in the interpretation of elemental ratios compared to the familiar "Redfield ratio" of 106C:16N:P. For the seagrass T. testudinum, the critical ratio of N:P in green leaves that indicates a balance in the availability of N and P is ca. 30:1, and monitoring deviations from this ratio can be used to infer whether N or P availabilities

Figure 2. Conceptual model showing the change in importance of primary producers as nutrient availability increases from low (oligotrophic) to high (eutrophic).	Figure 3. Conceptual model indicating how elemental ratios of seagrasses respond to increasing nutrient availability.

are limiting this species’ growth. Hence, T. testudinum is likely to be replaced by faster-growing competitors if nutrient availability is such that the N:P of its leaves is ca. 30:1. A change in the N:P in time to a value closer to 30:1 is indicative of eutrophication (Figure 3).

These models lead directly to a definition of trends likely to be encountered in the seagrass communities of south Florida if humans are causing regional changes in nutrient available because of alterations to quantity and quality of freshwater inputs to the marine ecosystem:

    1) regional eutrophication will cause N:P ratios of seagrasses to approach 30:1 from higher or lower values indicative of oligotrophic conditions; and

    2) regional eutrophication will cause a shift in species dominance in south Florida seagrass beds. The first responses to eutrophication will be evidenced by an increase in the relative abundance of fast-growing seagrass species (H. wrightii and S. filiforme) at the expense of the now-dominant, slow-growing T. testudinum. At later stages of eutrophication, macroalgae and microalgae will become the dominant primary producers.

At 4 nearshore Level 1 sites in the Florida Keys, there has been an increase in the relative abundance of macroalgae over the period 1995 - 2003 that is consistent with an increase in nutrient availability. At none of these has there yet been a decrease in seagrass abundance, but our conceptual model predicts that increases in fast-growing macroalgae should precede decreases in seagrass abundance (Figure 2). One example, from site 235 offshore of Lower Matecumbe Key, shows how macroalgae have steadily increased in abundance over the monitoring period (Figure 4). In addition to these sites where relative abundance of primary producers has changed, at 4 more Level 1 sites there have been long-term shifts in the ratio of nitrogen to phosphorus in seagrass leaves that are consistent with increases in nutrient availability.

Figure 4. A steady rise in the abundance of fast-growing macroalgae at some nearshore sites in the Middle and Lower Florida Keys is consistent with increase in nutrient availability at this site (Level 1 site 235, see figure 1 for location)

Figure 5. N:P ratios trending towards the balanced ratio of 30:1 indicate that light is becoming more limiting at this Level 1 site (number 239, see Figure 1 for location)