Multiple stressor effects on seagrasses in FL Bay: A mesocosm research approach
Marguerite Koch, Stephanie Schopmeyer, Claus Kyhn-Hansen
Florida Atlantic University, Boca Raton, Florida
Chris Madden
South Florida Water Management District, West Palm Beach, Florida
Although the South Florida rainy season can provide adequate fresh water flows to maintain mesohaline conditions in the Florida Bay estuary, a sub-tropical climate promotes frequent periods of drought and hypersaline conditions. A large-scale mesocosm facility was constructed at the FAU Gumbo Limbo Marine Lab (Boca Raton, FL) to experimentally define the upper levels of salinity tolerance for the dominant Florida Bay seagrasses (Thalassia testudinum, Halodule wrightii, and Ruppia maritima), using intact sediment cores in a highly controlled environment. In addition to establishing the upper levels of salinity tolerance for seagrasses in the Bay, the objective of the proposed study is to establish the relationship between hypersalinity tolerance and interactive stress variables, including high temperature, low light, and porewater sediment sulfide phytotoxicity. Results generated from the mesocosm experiments, and verified under field conditions, will be used to calibrate a Florida Bay seagrass simulation model. This model will be used to assess water management alternatives for Florida Bay Minimum Flows and Levels and CERP’s Florida Bay and Keys Feasibility Study.
The mesocosm design (Fig. 1) includes 16 three-meter diameter X three-meter height fiberglass tanks equipped with 2 powersweeps, one for circulation at canopy height, and the other for surface to bottom circulation and continuous aeration. Each tank has a 1,000 Watt metal halide light with a detachable ballast delivering ~900 µE m-2 s-1 light (PAR) to the water surface and ~650 µE m-2 s-1 to the canopy. The tanks hold 500 L of recirculating coastal seawater and the facility has temperature-control, maintaining the tank water temperature between 25-28 oC. The large number of tanks provides the opportunity to evaluate a broad range of salinity levels.
Figure 1. Mesocosm design showing tanks and light fixtures in upper right.
In the initial hypersalinity experiment, conducted from August to November 2002, eight different salinity treatments were examined: 35, 40, 45, 50, 55, 60, 65, and 70 PSU in replicate. Intact sediment cores (15 cm diameter X 20 cm depth) of the three seagrass species were collected from Whipray Basin to Garfield Bight (northcentral Florida Bay) and transported in coolers to the mesocosms. In addition to intact sediment cores, a second set of experimental units was collected with at least one apical rhizome shoot and 4-5 live short shoots for each species. At least two rhizome segments were transplanted into tubs (30 cm X 10 cm) with field sediment. These two collection approaches were utilized to account for any differences in seagrass stress response due to cut rhizomes in intact cores, a situation more likely problematic in cores of T. testudinum. In summary, each of the 16 tanks contained 3 intact cores and 3 tubs of T. testudinum, H. wrightii, and R. maritima.
To simulate the in situ rates of salinity increase in the northcentral Bay, salinity levels were adjusted at a rate of one PSU per day. The experiment was run as a closed system with salts (Instant Ocean) or deionized water amended, as needed, based on evaporation rates. Coastal seawater was also added weekly to maintain nutrient levels in the tanks. Tank salinity and temperature were monitored daily, while light, oxygen, and nutrient samples were taken weekly. The biological response variables monitored weekly-included T. testudinum leaf elongation rates and live shoot counts for all species in both cores and tubs. Plants were at salinity treatment levels for a one-month period, the limit of extreme hypersalinity conditions in northcentral Florida Bay. After this time, leaf photosynthetic performance was determined by chlorophyll fluorescence or photosynthetic quantum yield using a Diving PAM (Pulse Amplitude Modulation). Once plants were harvested, they were separated into leaves, roots, and rhizomes and immediately frozen in liquid nitrogen. Tissue was freeze-dried for determination of adenylates (ATP, ADP, AMP), carbohydrates, and proline (an important osmolite in seagrasses) using HPLC. Prior to freeze-drying, a sub-sample of leaf tissue was frozen separately for determination of total osmolality on a vapor pressure osmometer.
The slow in situ rate of salinity increase (1 PSU d-1) used in this study resulted in a highly linear response of total osmolality (mmol kg-1) to salinity treatment level (PSU) for T. testudinum (y = 166x + 1324 R2 = 0.97) and H. wrightii (y = 193x + 1848 R2 = 0.97). Total osmolality was high across salinity treatments for R. maritima (2783 to 3731 mmol kg-1) and no significant relationship was found between salinity level and osmolality (y = 52x + 2967 R2 = 0.19). The ability of all three seagrass species to osmotically adjust to increased salinity at rates observed in the field is supported by the fact that even at 70 PSU, live shoots were observed in all species. This was true even though the 70 PSU treatment took 30 days to come up to treatment salinity, and was >40 PSU for 60 days. While no mass mortality of shoots was observed in any treatment and species, species tolerance to hypersalinity varied among seagrass species. Interestingly, H. wrightii, frequently found in mesohaline conditions, appeared to be the most salt tolerant, increasing shoot numbers under all hypersaline treatments including 70 PSU. Leaves in the 70 PSU treatments were, however, beginning to exhibit chlorosis, quantified by the lower quantum yield of photosynthesis, as measured by PAM fluorescence. Live shoot numbers were stable for T. testudinum up to 60 PSU, but declined in the 65 and 70 salinity treatments, consistent with declines in leaf elongation rates. These salinity levels of tolerance for T. testudinum are higher than those previously observed in our preliminary hydroponic experiments without osmotic adjustment (50 to 55 PSU), suggesting the importance of a slow rate of salinity increase to seagrass tolerance under hypersaline conditions. In conclusion, the three dominant Florida Bay seagrass species examined in this study are highly tolerant of hypersaline conditions under in situ rates of salinity increase. With subsequent experiments, we will further examine the effects of interactive stressors (high temperature, low light, and sulfide phytotoxicity) on this adaptive potential of seagrasses to hypersaline conditions in the Bay.
Marguerite Koch, Aquatic Plant Ecology Lab, Biological Sciences Department, Florida Atlantic University, 777 Glades Rd, Boca Raton, FL 33431. Phone 561-297-3325; , Question 4