Once Is Not Enough: Seasonal Patterns in Abundance of Hydromedusae Are Not Consistent in the Coastal Waters of the Bay of Panama

Abstract

Synopsis Hydromedusae and other gelatinous zooplankton are poorly understood but important consumers in the plankton. Blooms of large-bodied medusae, which may have detrimental ecological impacts, have captured significant attention, yet the seasonal patterns of local abundance and diversity, and the factors that control them, are not well documented for the much more diverse tiny hydromedusae. There is virtually no published information on their phenology or ecology in tropical coastal systems. It has previously been suggested that increased productivity stimulated by upwelling promotes production of tiny hydromedusa and blooms as their abundance can be much higher during upwelling than during non-upwelling periods in the tropics. However, this conclusion was based on a single year of sampling in the Bay of Panama. To determine if a single year of sampling is sufficient to understand seasonal patterns in abundance and occurrence of blooms, we repeated this study 20 years later. Sampling 153 times between April 2022 and April 2023, we found similar genus-level diversity and overall counts of animals compared to the previous study, with medusae of Bougainvillia, Campalecium, Obelia, and Turritopsis accounting for 79% of the animals collected. However, we found that relative abundance was much higher during the non-upwelling season compared to the upwelling season, the opposite result compared to the previous study. These results highlight how strong baseline understanding of phenologies and the ecological conclusions drawn from them must be based on multiple years of data, even in regions with very strong seasonal changes in environmental conditions. In the case of the upwelling regions of the Tropical Eastern Pacific, further research is necessary to understand the generalizability of these patterns and to determine if contrasting results are due to inter-annual differences in oceanographic drivers or changing environmental baselines.