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Keywords:

  • Disturbance;
  • diversity;
  • long-term;
  • persistence;
  • sediment deposition;
  • sponge assemblages;
  • stability;
  • wind stress

Abstract

The importance of a long-term ecological perspective is well documented, yet long-term data are not widely available. This paper represents the first quantitative description of sublittoral sponge assemblages over long temporal scales (6 years) along the coast of the East Pacific Ocean (Mazatlan Bay), with the goal of describing their levels of intra- and inter-annual variability, and their relationship to changing environmental conditions. It was possible to detect an apparent short-term pattern (intra-annual), with a highly diverse and stable structure, usually in the drought season, which was consistent most of the years. However, only a few species showed regular (predictable) seasonal cycles. The species per square meter (from 0.1 to 0.5), total species per sampling (14–21), and H’ diversity using loge (1.4–2) also fluctuated greatly between years, suggesting that although a consistent short-term pattern was found most of the years, the inter-annual variability was also high. The univariate and multivariate results and regression models indicated that local winds played a key role in this short-term pattern (intra-annual). During the rainy season, or during the transition between the drought and rainy season, the winds coming from the southwest (WSW) produced an increase in the net sediment movement, which was an important factor for diversity. The long-term fluctuations (annual pattern of diversity) also correlated positively with wind speed (88% of the variance observed) and with sediment deposition (69% of the variance observed). In addition, the results indicated the existence of large-scale structuring factors, as the annual pattern of diversity also was correlated positively with the Southern Oscillation (SOI) and Multivariate ENSO (MEI) indices (82% and 88% of the observed variance, for MEI and SOI, respectively). In conclusion, this study suggests that sponge assemblages in Mazatlán Bay are extremely dynamic, and changes in this community are associated with processes that occur over the short- and long-time scales (several months to several years). The significant positive relationship between wind speed and sediment deposition showed that the main factors controlling the diversity in these shallow rocky ecosystems are the winds and the swell. A high proportion of coarse sand also indicated a very high energy in the environment, which, combined with the effect of silting, abraded and removed sponges and other organisms. These features contribute to the instability of the community by producing dramatic fluctuations in species abundance and preventing competitive processes from producing a more stable community.