Continuous Light Monitoring in Florida Bay: Interannual Variations and Light Availability

Continuous Light Monitoring in Florida Bay: Interannual Variations and Light Availability to Seagrasses

Laura A. Yarbro and Paul R. Carlson

Florida Fish and Wildlife Conservation Commission, St. Petersburg, FL

Continuous light monitoring, water quality, and epiphyte assessment in Florida Bay provide the framework for modeling the responses of seagrass communities to historical and anticipated changes in water clarity associated with recent changes and future management, respectively. At this time, we have collected four years of data (over 600,000 data records), and we can begin to see long-term as well as short-term temporal trends in the data. Furthermore, as the Comprehensive Everglades Restoration Plan (CERP) begins implementation, we see the utility of continuous light monitoring data to evaluate impacts of changed water management on Florida Bay.

Continuous light data have been collected at seven stations: Johnson Key, Rabbit Key, Rankin Lake, Little Madeira Bay, Long Sound, Butternut Key, and Peterson Key. Data collection began in November 1998 and continues at present. At each of these seven stations, we have installed Licor LI-1400 data loggers connected to two spherical, underwater light probes. The probes are PAR (photosynthetically-active radiation) sensors, which measure light with wavelengths from 400 nm to 700 nm. Their spherical shape integrates downwelling light and some bottom reflectance in much the same way that seagrass blades receive light form all directions. The probes are mounted on PVC staffs. In addition to measuring the amount of light available to seagrass and phytoplankton at the surface and at the bottom, the two probes enable us to calculate the diffuse attenuation coefficient for light in the water column. The logger records data from each probe every 15 minutes from 0530 h to 2100 h EST each day. Probes are cleaned, and data downloaded twice monthly.

In addition to the continuous bottom light data, we have summarized data by calculating several key parameters. Daily total PAR flux and peak mid-day PAR values, measured between 1100 h and 1300 h, have been calculated for each site using SAS (SAS Institute, 1988). We have also calculated the number of hours in each day when PAR values have exceeded 200 and 500 uE, respectively called Hsat 200 and Hsat 500. These parameters are based on the work of Dennison and Alberte (1985) who found that Zostera survival and growth were positively correlated with the number of hours each day when light exceeds the saturating light intensity for eelgrass, and they called the parameter Hsat. Our choice of two Hsat values, one based on 200 uE and the other on 500 uE, reflect the range of Hsat values which might be applied to seagrasses. Monthly averages have been calculated for all parameters.

We have also collected discrete water samples monthly at each monitoring site and 12 other sites scattered across Florida Bay for analysis of total suspended solids, turbidity, color and chlorophyll. For the first two years of the study, Thalassia shoots were collected monthly at each monitoring site and shoot morphometrics and epiphyte loading were measured on these samples.

All four measures of PAR—daily sum PAR, Hsat 200, Hsat 500 and mean mid-day PAR – varied significantly seasonally and among sites (Figure 1). Mean daily photon flux was generally higher at Butternut and Madeira than at Long Sound, Peterson Key or Rabbit Key. Peterson Key values have not been corrected for significant morning shading. Lowest photon fluxes occurred at Johnson Key and Rankin Lake. The seasonal pattern of daily photon flux was similar for all sites. Highest bottom light values occurred in the months of March, April and May of both 1999 and 2000. Lowest values occurred in the months of October through December. Low values coincided with seasonal insolation minima, but the maximum values in spring probably resulted form a combination of cool water (low phytoplankton growth rates), low rainfall and low cloud cover. Low light values in the fall also coincided with near maximal water temperatures. The lag between declining water temperatures and declining photoperiod in the fall is a potentially major factor contributing to historical and ongoing seagrass mortality.

Figure 1. Average mid-day Photosynthetically-active radiation
at seven Florida Bay sites.

Although Tropical Storm Harvey and Hurricane Irene did not physically disrupt or defoliate seagrass communities in fall 1999, the resultant drop in water clarity lasted for five months and had a significant impact on seagrass communities, especially in western Florida Bay (Figure 1).

With long-term data sets, subtle annual differences appear (Figure 2). Fall daily photon flux values fell between 1998 and 1999 and have shown successive increases in 2000, 2001 and 2002. In contrast, spring photon flux values declined each year since 1999, declining almost 30% over the three year period. With long–term data sets, the effect of climatic factors such as El Nino and drought years on the seasonal balance of light parameters might emerge. Their combined effects on seagrasses in Florida Bay might be considerable.

Figure 2. Daily photon flux averaged by season for all monitoring sites in Florida Bay.

We thank the following project staff: Paul Hunter, Jeff Absten, Alice Ketron, Herman Arnold, Braxton Davis, Kevin Madley, Brad Peterson, Jenny Davis and Manual Merello. The U.S Geological Survey and Everglades National Park provided financial and logistical support.

Laura Yarbro, Florida Fish and Wildlife Conservation Commission, 100 Eighth Ave. SE, St. Petersburg, FL 33701, Phone: 727-896-8626, Fax 727-823-0166, , Question 4.