chapter  6
20 Pages

Relationship between Macroinfaunal Diversity and Community Stability, anda Disturbance Caused by a Persistent Brown Tide Bloomin Laguna Madre, Texas

A brown tide, Aureoumbra lagunensis, bloom persisted throughout the 1990s in Baffin Bay and upper Laguna Madre, Texas, which resulted in increased phytoplankton biomass and decreased bottom light levels. Coincidently, changes in benthic macroinfauna abundance, biomass, and diversity were studied from 1988 to 2000. The combination of the bloom and benthic study allows us to perform a post hoc analysis to determine if the brown tide affected macroinfauna. Three hypotheses were tested: (1) initial disturbance leads to a diversity loss, (2) more diverse systems resist change, and (3) more diverse systems recover more rapidly. Four stations were sampled quarterly, two in Baffin Bay in an open bay, muddy bottom habitat, and two in Laguna Madre in seagrass and an

Abstract .......................................................................................................................................... 115 6.1 Introduction .......................................................................................................................... 116 6.2 Materials and Methods ......................................................................................................... 117 6.3 Results ................................................................................................................................... 119

6.3.1 Water Column ........................................................................................................... 119 6.3.2 Macrofauna ............................................................................................................... 121

6.3.2.1 Abundance ................................................................................................. 121 6.3.2.2 Biomass ...................................................................................................... 122 6.3.2.3 Diversity ..................................................................................................... 123

6.3.3 Community Structure ...............................................................................................124 6.4 Discussion ............................................................................................................................. 128 Acknowledgments .......................................................................................................................... 132 References ...................................................................................................................................... 132

adjacent unvegetated habitat. Approximately 75% of invertebrate biomass was lost in both Baffin Bay and upper Laguna Madre prior to the brown tide. This loss is linked to the high salinities in the bays from 1988 to 1990. In addition, freezes in December 1989 to January 1990 caused two large fish kills and probable invertebrate kills. These die-offs resulted in increased ammonium in the system, which fueled the brown tide bloom. The loss of macroinfauna decreased the grazing potential of benthos on the brown tide organism. Both systems revealed decreases in abundance, biomass, and diversity concurrent with the increased concentrations of brown tide. The upper Laguna Madre seagrass stations, with the highest diversity, recovered in abundance and diversity during later dates, while the muddy bottom habitats, with low diversity, did not. On the other hand, biomass recovered at the muddy Baffin Bay stations, but not in the seagrass bed. Thus, the effect of brown tide on the macroinfauna is dependent on the habitat and diversity. Community structure changed with the onset of brown tide. Species diversity decreased and disturbance tolerant species, such as Streblospio benedicti and Prionospio heterobranchia, increased dominance in the system. There appear to be two different equilibrium states in this ecosystem: one prior to brown tide and one after onset of brown tide. The new equilibrium state is less diverse and lower in abundance and biomass. The effect of brown tide on the macroinfauna is important because the benthos, in Baffin Bay and Laguna Madre, constitute an important food source for commercial and recreational fish and play a role in the regeneration of nutrients to the shallow water ecosystem.