For the estimation of the benthic primary productivity at the ambient light conditions, photosynthesis-irradiance (PI) curves are measured that link light intensity to production at each of the above mentioned five stations. We set up a system for the measurement of PI curves in Gulf of Mexico sediment, which we collect on site. This analysis can be conducted while the sediment is still contained in the core liners thereby minimizing disturbance of the cores. For these measurements, the cores with overlying water are closed gas-tight by a transpired lid and water is circulated at in-situ temperature through the water-filled head space of the core liner. The core surface then is exposed to different light intensities over the range measured in-situ. Oxygen production by the microphytobenthos community colonizing the surface of the sediment core then is measured in the circulating water flow using an oxygen microoptode. By alternating dark and light cycles, production and consumption rates of the sediment can be assessed. So far cores from the nearshore site have been analyzed. In year 3 we propose to install four CT divers that log conductivity and temperature near the surface and at medium depth at stations A and B in order to collect a minimum of density data for the water column at these stations which would improve the modeling of the water column transport processes in the region.
In regards to our study of the growth rate and diet of early juvenile gag , in the 2008 field season (April to mid-July, Table 1b), Stefan Bourgoin (Thistle's graduate student) collected in two seagrass meadows in the vicinity ofTurkey Point Shoals in the northeastern Gulf of Mexico (Fig. 3). During the same period, Stacey Harter (Andrew David's technician at the NOAA NMFS office in Panama City) collected in three seagrass meadows in St. Andrew Bay. To assess weather conditions, we deployed instruments at Turkey Point Shoals and at St. Andrew Bay that recorded temperature, salinity, and light at a hourly intervals. Instruments were swapped out at biweekly intervals for cleaning and data recovery. At the end of the field season , we began work on the fish from St. Andrew Bay and from Turkey Point Shoals. To obtain growth rates, we are measuring each specimen's weight and determining its age from the growth rings on its otoliths (see e.g., Stelcheck et al., 2003). To assess its diet at the age and on the day it was caught, we are quantifying the amount of each prey type in its guts (gag individuals swallow their prey whole, Mullaney and Gale, 1996).
James Nelson (Advisor: J. Chanton and C. Koenig) completed his work determining the maximum turnover rates for gag liver, gonad, and muscle tissue. The results of this work validated that his approach could be applied to a field study. Nelson is currently focusing on collection of offshore species and has conducted 4 collection trips in late 2007/2008. The hypothesis we are testing is that seagrass nearshore benthic production supports the spawning of offshore species by the egress of pinfish and pigfish to deeper waters in the fall. We propose to continue these collections to quantify annual variability, which we expect to see. We are also working with Dr. DeVries (NOAA-NMFS Panama City Lab) who has collected a number of offshore species on which we have conducted stable isotopic analysis.
We also started a project to characterize the faunal communities associated with seagrass beds across the entire BBR (St. Marks to Tarpon Springs). Although the BBR is home to one of the largest and most pristine seagrass beds in the world, very little is known about absolute abundances and spatial distributions of juvenile gag and their potential prey across it. Using a generalized random-tessellation stratified (GRTS) design, a spatially-balanced approach (Stevens and Olson 2004), we are sampling with beam trawls across the entire BBR seagrass beds (Fig. 14). This study will define the spatial extent of the BBR seagrass bed, characterize the composition of seagrass and faunal communities, and elucidate potential mechanisms driving the patterns (e.g., influence of saltmarsh and other adjacent terrestrial habitats). Importantly, the GRTS design will allow for far more precise estimates of absolute abundances of gag and other ecologically-important species (e.g., gag prey) than random or systematic designs due to the local variance estimator it employs. We have secured additional funding from the Florida Fish and Wildlife Conservation Commission to achieve this sizable task.