The effects of a red tide, Karenia brevis episode on the benthic macroinvertebrate communities of South Padre Island, Texas
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South Padre Island, Texas has experienced increasing frequency of harmful algal blooms (Karenia brevis) since the 1900’s, many of which resulted in massive fishes kills. In addition, shorebirds and other terrestrial wildlife deaths have been linked to K. brevis blooms prompting investigations of potential pathways by which brevetoxins may be transmitted to higher trophic levels. The benthic macrofanual community inhabiting the swash zone of sandy beaches consists primarily of filter feeding invertebrates which may serve as toxin bioaccumulators to secondary consumers of both marine and terrestrial food webs. The first reported mass mortality of swash zone benthic macrofauna on the south Texas coast associated with a harmful algal bloom, occurred on the shoreline of Boca Chica beach in 2005. In September 2011, South Padre Island was impacted by a four month long red tide event, killing a total of 4.4 million fishes. The effects of K. brevis on benthic communities are largely undocumented and the few studies that have been conducted often produce conflicting results. Thus, the purpose of this study was to assess the effects of K. brevis on the benthic macroinvertebrate community following the 2011 harmful algal bloom. Sampling was conducted from six 400 m stretches of beach prior to and after a prolonged period (4 month) of high red tide cell count concentrations. Species identification, abundances, and biomass were obtained from each sample. A oneway ANOSIM indicated that the pre and post bloom community structures were significantly different in terms of both abundance and biomass. Pre and post harmful algal bloom assemblages were 68.5% and 67.0% similar with respects to species relative abundances and biomass. The differences in communities were contributed to greater abundances and biomass of Scolelepis squamata in the pre-bloom samples and greater abundances of Donax variabilis and Ancinuss depressus in the post-bloom samples. Multi-dimensional scaling analyses also indicated a distinct separation of the before and after red tide communities in both abundances and biomass. However, when compared with data from a previous study, the variation among pre and post bloom communities was similar suggesting that seasonal variation rather than the effects of K. brevis may account for the differences between pre and post bloom communities. Though there is speculation that secondary effects of blooms, specifically anoxia, may be the underlying cause for mortalities in benthic invertebrates, other potential alternatives such as different Karenia spp. and levels of toxicity may be associated as well. Nevertheless, the results of this study pose a question of why two red tide events, 2005 and 2011, similar to an extent, resulted in two very different outcomes with regards to the benthic macrofauna community.