Bay Scientists Rediscover the Obvious

New study helps explain recent scarcity of bay nettles in the Chesapeake Bay

Wondering why you saw so few jellyfish in the Chesapeake Bay this summer?

Bay Sea Nettle Chrysaora chesapeakei

Well, no. We've known that Sea Nettles require at least moderate salinity for a long time; at least as long as I've been living in the area, and probably longer. I know of at least one long-term study of Sea Nettle abundance which long pre-dated my arrival, and we all just "knew" that in low salinity years, they started later, and were much less abundant.

A new, long-term study of how environmental conditions affect the abundance and distribution of bay jellies helps explain the widely reported scarcity of bay nettles during the past few months and raises concerns about how a long-term continuation of this trend might harm bay fisheries as climate continues to warm.

The research, reported in the latest issue of Estuaries and Coasts, used two long-term monitoring surveys to study the interplay between jelly populations and water temperature, salinity, oxygen levels and other factors. Compiling and analyzing population data for five different jelly species in the years between 1984 and 2012, it is the most comprehensive study of Bay jelly populations ever undertaken.
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The two most common species in the study, the bay nettle Chrysaora chesapeakei and the comb jelly Mnemiopsis leidyi, are both transparent, jellylike predators that drift with the currents, but differences in their size, anatomy, feeding behavior and life cycles translate into stark differences in their population dynamics and impacts on the Chesapeake ecosystem.

For Chrysaora, a stinging jelly painfully familiar to many Bay enthusiasts, the team confirmed and further quantified a preference for warm, relatively salty waters—recording peak abundances between July and September in waters above 70 degrees, and at salinities of 16 parts per thousand (bay waters range from near zero ppt where tributaries enter to around 32 ppt—as salty as the ocean—near the bay mouth).

Mnemiopsis leidyi

Confirmed and quantified! Ah!

The bay nettle's preference for saltier water helps explain widespread reports of its scarcity this summer in many of the Chesapeake Bay tributaries it typically inhabits, as freshwater runoff from record rains drastically lowered their salinity. It was particularly wet in May—the rainiest in Richmond since record-taking began there in 1872—just when the nettles' bottom-dwelling life stage begins to bud off its tiny, sensitive, free-floating medusae.

And with good luck, maybe it killed off enough medusae that they will be slow to take off next year too.