Factors affecting distribution and abundance of jellyfish medusae in a temperate estuary: a multi-decadal study

Three hydrozoan species, reputedly from the Black Sea ( Maeotias marginata , Blackfordia virginica , Moerisia lyonsi ), are now found throughout the San Francisco Estuary, California, but long-term and seasonal patterns of distribution and abundance have been poorly documented. We evaluated trends from 35 years of monthly otter trawl data and from a 2-year macrozooplankton survey in Suisun Marsh, a brackish region with extensive tidal sloughs and channels that is part of the San Francisco Estuary. Medusae of all three hydrozoans occurred primarily during the dry season (summer–fall). Abundance of M. marginata medusae significantly increased since the 1980s. Moerisia lyonsi was the most abundant hydrozoan in the macrozooplankton medusa survey followed by M. marginata and B. virginica. Salinity and temperature were strongly positively associated with medusa abundance. Maeotias marginata occurred in the lowest salinity range (2.3–9.1 ppt), while M. lyonsi (2.8–9.9 ppt) and B. virginica (5.6–10.3 ppt) occupied slightly higher salinities. Overall, abundance and distribution of medusae of these three hydrozoans in Suisun Marsh depended on seasonal stability of environmental conditions that favored blooms. While harmful effects have yet to be demonstrated, they could become more of a problem as both sea level and water temperatures rise, especially given the combined range of environmental conditions at which the three species occur.