Multiscale Assessment of the Morphometrics and Demography of Thalassia testudinum (Turtlegrass): A New Approach to Ecosystem Assessment. Florida Department of EnvironmentalProtection/ US EPA.

SCALE-BASED VARIABILITY IN Thalassia testudinum MORPHOLOGY AND DEMOGRAPHY ALONG THE WEST COAST OF FLORIDA.

Durako¹, M. J., M. Merello²,
G. McRae², P. R. Carlson², Jr.
¹The University of North Carolina at Wilmington, Center for Marine Science Research, Wilmington, NC 28403; ²Florida Marine Research Institute, St. Petersburg, FL 33701.

ABSTRACT

Hierarchical sampling based on tesselated hexagons was used to examine spatial dependence among potential ecoindicators for the seagrass Thalassia testudinum at four Florida Gulf coast sites. Sampling grids consisted of three concentric 30-cell tesselations. The finest level of resolution was located nearest to suspected anthropogenic or natural perturbations. Sample stations were randomly located within each grid cell. A suite of morphometric and demographic characteristics were measured at each sample station. All characteristics varied significantly at the regional (among-site) scale. In 1997, within-site scale-based variability in shoot-density was significant only at the Tampa Bay site. Sampling at this site was centered on a shallow area frequently traversed by small-vessels. Samples from the small-scale sampling grid had significantly lower short-shoot densities, but similar apical densities to those of the large-scale grid, suggesting that the effect of vessel traffic is quite localized at this site and that Thalassia may exhibit a compensatory increase in branching in response to disturbances that reduce shoot densities. Shoot specific characteristics such as the number of leaves short-shoot^-1 and leaf width exhibited the least spatially-dependent variability within each of the four sites; the numbers of leaf scars short-shoot^-1 exhibited the greatest spatial variability.

BACKROUND

Thalassia beds are of prime importance to the trophic dynamics and economy of many coastal and estuarine regions in the Gulf of Mexico. The shallow depth distribution and relatively high light requirements of Thalassia beds place them in close proximity to the land/sea margin. Thus, Thalassia beds are subject to natural and human-induced stresses and disturbances associated with this boundary.

Because Thalassia habitats form long-lived, structurally-complex benthic communities, there has recently been an expanding interest in evaluating various structural and dynamic parameters (ecoindicators) of this species as a means for assessing estuarine trophic state and ecological condition of estuaries and coastal areas around the Gulf.

OBJECTIVES

1) Examine the relative significance of fine-to-larger scale variation and spatial structuring, in distribution, abundance, morphometric, and demographic characteristics of the dominant coastal seagrasses in the Gulf of Mexico.

2) Determine if any of the measured characteristics can be applied as robust indicators of population and ecosystem health, and through an examination of within-scale, among-scale, and scale-by-site interactions associated with these data.

SAMPLING DESIGN

A hierarchical sampling design based on tesselated hexagons was used to examine spatial dependence among the potential ecoindicators (Figure 1). The hierarchical grids consisted of three 31-cell symmetric tesselations (approximate scales: 100s m² hexagon^-1 , 10,000s m² hexagon^-1, and 1,000,000s m² hexagon^-1), with the finest level of resolution located near anthropogenic or natural perturbations, or in the absence of pertubation, in the center of a meadow.

METHODS

The primary sampling effort was conducted over a four week period in the late-summer early-fall at all sites (semi-synoptic). At each sample station, a 15 cm diameter core sample was obtained. Core samples were rinsed free of sediments using water pressure, with care taken to ensure that short-shoots remained attached to rhizomes. Plant material was placed in labeled whirl-pak bags, and frozen at -4 C for subsequent analysis.

Thalassia Morphometrics: For each short-shoot, the number, lengths and widths of green leaf blades was determined. Leaf and sheath material was then peeled away from each short-shoot, and the number of leaf scars (plastochrone intervals) and stem length recorded.

Sample Sites

Charlotte Harbor/ Color/nutrients/salinity. Charlotte Harbor is a shallow bay system that receives freshwater from three major rivers. Large portions of the western side of the Harbor frequently experience low-salinity (< 15 ppt) and low-water transparency (<10% surface irradiance) conditions. A gradient in Thalassia abundance currently exists from sparse beds at the north to increasing abundance southward. This gradient may be due to a combination of reduced salinties, increased nutrient input, and high light attenuation due to both high color of the freshwater inflow and the precipitation of dissolved organic matter when the fresh river water mixes with estuarine water.

Tampa Bay/ Physical impact. Many anthropogenic impacts have reduced or degraded the seagrass habitats within Tampa Bay with an aggregate loss of approximately 80% of the historical distribution. The running of small motor vessels and trawling for shrimp in the shallow seagrass beds in the southern bay have been suggested as a current negative impacts. This area was sampled to determine if there is a spatial gradient in the condition of Thalassia populations related to propellor or trawl impacts.

Anclote River / Power plant cooling water effluent a point source of thermal stress. Florida Power Corporation's Anclote River Energy Complex is located just inland from the shore of the Gulf of Mexico on the Anclote river. Heated water is discharged into a dredged discharge canal located north of the River. Sampling at this site used the the point of discharge as the central point of sampling and radiate outward from this defined source of stress.

Homosassa River Mouth/point-source riverine nutrient input into a relatively pristine ecosystem. The Homosassa River is a spring-fed river that empties freshwater into "Big Bend" area. Most urban and rural development in the region currently uses on-site septic systems and recent studies indicate they are not performing adequately to treat household wastes. Sampling at this site focused on the River mouth and navigation channel extending to the west as the centroid of nutrient inputs from the watershed.

SUMMARY OF RESULTS

Among-site scale variability was greater than among-scale based variability for all the density characteristics (see figure above and compare p values in Tables 1 and 2).

In 1997, among-scale based variability in shoot-density was significant only at the Tampa Bay site. The large-scale samples had significantly higher densities than the small scale samples. Apical density also exhibited significant scale-based variability, but the small-scale samples actually had densities comparable to the large scale samples even though the shoot densities were significantly lower, suggesting that an increase in branching may be a response to prop scarring or trawling. Semivariograms of density  indicated that scale-dependent variability in shoot density increased with the scale of sampling as indicated by the increase in the sill and range.

In 1998, among-scale based variability in shoot-density was significant at the Tampa Bay and Charlotte Harbor sites for Thalassia. In Tampa Bay, the small-scale samples again had the lowest densities. In Charlotte Harbor, the large-scale samples had significantly lower densities than the small and medium-scale samples, possibly reflecting the larger salinity gradient. Thalassia apical density also exhibited significant scale-based variability in Tampa Bay, but this year the medium-scale samples had significantly higher densities than the small scale samples.

Among-site scale variability was generally greater than among-scale variability for all the morphometric characteristics (compare p values in Tables 3 and 4).

In 1997, all morphometric characteristics varied significantly at the regional scale. The Tampa Bay populations had the highest mean values for four of the five characteristics compared: max leaf length, leaf width, shoot-specific leaf area, and leaf scars per short-shoot. In contrast, within site, scale based variability in the five morphometric characteristics exhibited no consistent pattern. Shoot age (leaf scars per short-shoot) exhibited significant scale-based variability at all four sites, while # of leaves per short-shoot did not exhibit any significant scale based variability.

In 1998, the two southern sites had significantly higher # of leaves per short-shoot than the two northern sites. The Tampa Bay site had the longest and widest leaves, and thus the highest shoot-specific leaf area index, compared to the other three sites. The Tampa Bay site also had the highest shoot ages, which may explain why these shoots were larger. The number of leaves per short-shoot and maximum leaf length exhibited significant among-scale variability at both the Charlotte Harbor and Tampa Bay sites. Only the scale extremes were significantly different, i.e., either the small scale or large scale samples. The medium-scale samples never had morphometric characteristics that were significantly different than both the larger and smaller-scale samples.

 

SUMMARY

1. For the four sites sampled along the west coast of Florida, Thalassia abundance significantly varied at the regional (among-site) scale.

2. Within-site scale-based variability in Thalassia shoot-density was significant only at the two southern sites.

3. Thalassia apical densities also exhibited significant scale-based variability at the Tampa Bay site; the small-scale samples had the lowest apical and short-shoot densities.

4. Variability in Thalassia’s morphometric characteristics was greater at the regional (among-site) scale than at the local, within-site scale.

5. Shoot-specific characteristics such as the number of leaves per short-shoot (CH and TB), maximum leaf length (CH, TB, and HR), shoot-specific leaf area (TB and HR) and numbers of leaf scars per shoot (HR) exhibited among-scale variability.

6. Leaf width exhibited the least spatially-dependent variability within each of the four sites.

7.  In 1997, Thalassia testudinum was present at all four sites. Thalassia was most abundant at the Tampa Bay and Homossassa River large scale sites and least abundant at all three scale at the Anclote River site.

8.  In 1998, Thalassia was present at all sites and at all scales. Thalassia was most abundant at Homossassa River and Tampa Bay and least abundant at Anclote River. The Tampa Bay site had significantly higher overall densities of Thalassia apicals than the three other sites, and except for the Homossassa River site, apical densities correlated with each site’s short-shoot densities across scales.

Support for this study provided by the USEPA, Grant #R825145-01-0