Factors influencing appearance, disappearance, and variability of abundance of the sea nettle Chrysaora quinquecirrha in Chesapeake Bay
Sexton, Margaret Augustine
Hood, Raleigh R
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Some evidence suggests that jellyfish populations are increasing globally. Because of their nuisance to humans and trophic interactions, it is desirable to identify variables that control jellyfish blooms. Chrysaora quinquecirrha (Desor 1848) medusae are seasonally abundant in the mesohaline Chesapeake Bay. Efforts to predict medusa abundance have relied upon visual counts to estimate medusa abundance. A comparison of visual counts to vertical net hauls showed that visual counts underestimate abundance as compared to vertical net hauls, but the two measures captured the same trend 63% of the time. Smoothing improved agreement between estimates of abundance made by the two methods. The optimal moving average window size of 5 observations indicates that visual counting can be used to examine short term variability in abundance. Previous efforts to identify variables that control the annual medusa bloom have traditionally focused on the magnitude of the peak in medusa abundance, but the timing of the bloom also varies from year to year. The timing of first appearance, peak abundance, and disappearance of medusae are examined for correlation with environmental conditions. Streamflow, which influences temperature and salinity, particularly in the first six months is an important driver of the timing of the medusa bloom; however, relationships between the timing of first appearance and both timing and magnitude of the peak indicate that the biology of the polyp stage needs to be considered in order to improve the predictability of the annual medusa bloom. C. quinquecirrha medusae have a patchy distribution, even at times of high abundance and under optimal conditions. Two cyclic patterns in variability of abundance over short time scales were identified whose periods correspond to the lunar fortnightly constituent of the tidal force and the time between successive peaks in strobilation by the polyps. Apparent changes in abundance at the surface are caused by changes in depth distribution, which is influenced by water temperature and wind speed. Finally, a laboratory experiment showed that low temperatures cause medusae to sink before cooling to the limit of their physiological tolerance and may have implications for deposition of organic matter associated with seasonal disappearance of medusae.