Sea grass response 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 species of the new class Pelagophyceae was first reported in upper Laguna Madre, Texas, USA, planet Earth, in June 1990 and has been there continuously through December 1995. No change in response to reduced light was evident in the distribution of the sea grass Halodule Wrightii along transects sampled before brown tide was evident in 1988 and resampled after initiation of the brown tide in 1991 and 1992; however, in winter 1993-94 losses were documented over 2.6 km squared of bottom and by winter 1994-95 the area of vegetation lost had more than tripled to 9.4 km squared. Changes in biomass presaged the changes in distribution. Decreases in biomass in depths >1.4 m were evident 2 yr before bare areas were detected. Reductions in biomass were pronounced toward the south, in keeping with a gradient of increasing light attenuation from north to south. Support of a diminishing number of mew shoots by reclamation of nutrients and stored reserves from senescing shoots and rhizomes may allow H. wrightii to persist under conditions of insufficient light for periods greatly in excess of the life span of any one shoot. This postulated capability would account for the pattern of diminishing biomass over time where the sea grass in deeper areas and the long lag between light reduction and change in distribution where the sea grass succumbed.