Primary Production Potential of Blue-Green Algal Mats on Southern Texas Tidal Flats

Abstract

Blue-green algal mat biomass, productivity, and nitrogen fixation were determined for two southern Texas tidal flats, and environmental factors affecting productivity rates were assessed. Algal biomass over three seasons ranged from 0.50 to 1.53 g chl a · m-2. The rate of carbon fixation (by '4C technique) ranged from 0.6 g C'm-2 day-' (in fall) to 2.4 g C'm- 2day-' (in spring) on days when flats were flooded. The rate of N2 fixation on the same days varied from 0.09 to 0.5 kg N-ha-'day-'. On days when mats were severely desiccated (mat water 25% but < 60%) were capable of fixing gaseous CO2 in air. Exposure to sunlight equivalent to one-fourth full sunlight (ca. 500 uE'm-2s-') sustained maximum photosynthetic activity. Tidal flats consist of broad, relatively level shorelines of bays, estuaries, or lagoons that are periodically inundated by high tides and storm surges. In the Laguna Madre of southern Texas, this habitat typically comprises the entire leeward barrier island intertidal zone and covers hundreds of km2 (Fig. 1). While an arid, windy climate, high summer temperatures, and subsequent hypersalinity prevent establishment of estuarine salt marsh (Simmons, 1957; Copeland, 1966; Hoese, 1967), these flats are often covered with a 2-4 mm thick blue-green algal mat. Quantitative productivity data for such subtropical Texas mats are unreported. Existing studies of benthic microalgae production have dealt mostly with temperate mudflat diatom communities (Pomeroy, 1959; Pamatmat, 1968; Riznyk et al., 1978) or salt marsh systems containing sparse blue-green mats (Gallagher and Daiber, 1974; Van Raalte et al., 1976). Sage and Sullivan (1978) found very few blue-green mats in a Mississippi salt marsh, and they concluded that mat distribution in eastern Gulf Coast marshes was limited by shading from salt marsh vascular plants. Wanders (1976), as part of a study on tropical coral reefs, found high primary productivity for epilithic blue-green algae, but only over limited areas. Recent work on blue-green mats from southern California marshes (Zedler, 1980) revealed that these subtropical flats had much higher productivity than temperate flats; and blue-green production was equal to the highest values recorded for some mudflat diatoms (Riznyk et al., 1978). Dawes et al. (1978) concluded that intertidal green and red macroalgae were adapted to periodic tidal exposure and were more productive when exposed to air than when continuously submerged. Dawes (1981) emphasized the need for additional studies of production potential of intertidal algae and evaluation of the concept that such algae are under stress due to exposure per se. During the present study, primary