SEAGRASS RESPONSES TO LONG-TERM LIGHT REDUCTION BY BROWN TIDE IN UPPER LAGUNA-MADRE, TEXAS - DISTRIBUTION AND BIOMASS PATTERNS

A brown tide caused by a very dense bloom of an as yet undescribed spe cies of the new class Pelagophyceae was first reported in upper Laguna Madre, Texas, USA, in June 1990 and has been there continuously throu gh December 1995. No change in response to reduced Light was evident i n the distribution of the seagrass Halodule wrightii along transects s ampled before the brown tide in 1988 and resampled after initiation of the brown tide in 1991 and 1992; however, in winter 1993-94 losses we re documented over 2.6 km(2) of bottom and by winter 1994-95 the area of vegetation lost had more than tripled to 9.4 km(2). Changes in biom ass presaged the changes in distribution. Decreases in biomass at dept hs >1.4 m were evident 2 yr before bare areas were detected. Reduction s in biomass were more pronounced toward the south, in keeping with a gradient of increasing light attenuation from north to south. Support of a diminishing number of new shoots by reclamation of nutrients and stored reserves from senescing shoots and rhizomes may allow H. wright ii to persist under conditions of insufficient light for periods great ly in excess of the life span of any one shoot. This postulated capabi lity would account for the pattern of diminishing biomass over time wh ere the seagrass persists in deeper areas and the long lag between lig ht reduction and change in distribution where the seagrass succumbed.