Long-term time-series study of salp population dynamics in the Sargasso Sea

Abstract

Salps are bloom-forming, pelagic tunicates with high grazing rates on phytoplankton, with the potential to greatly increase vertical particle flux through rapidly sinking fecal pellets. However, the frequency and causes of salp blooms are not well known. We quantified salps from day and night zooplankton net tows in the epipelagic zone of the North Atlantic subtropical gyre as part of the Bermuda Atlantic Time-series Study (BATS). Salp species and size were quantified in biweekly to monthly tows from April 1994 to November 2011. Twenty-one species of salps occurred at the BATS site over this time period, and the most common bloom-forming salps were Thalia democratica, Salpa fusiformis, Weelia (Salpa) cylindrica, Cyclosalpa polae, and Iasis zonaria. Five species of salps exhibited diel vertical migration, and salp abundances varied seasonally, with T. democratica, S. fusiformis, and C. polae blooms coincident with the spring phytoplankton bloom, and W. cylindrica blooms occurring more often in late summer. For T. democratica, mean annual biomass increased slightly over the time series and was elevated every 3 yr, and biomass increased in the presence of cyclonic mesoscale eddies. Decadal climate oscillations and biogeochemical conditions influenced multi-year trends in salp abundance and biomass. Both total salp and T. democratica abundance were positively correlated with primary production, total salp biomass was positively correlated with the North Pacific Gyre Oscillation, and T. democratica biomass was negatively correlated with the Pacific Decadal Oscillation. These salp bloom dynamics have important implications for planktonic food web interactions and biogeochemical cycling.

Salp contributions to vertical carbon flux in the Sargasso Sea

“…Interannual variations in mesozooplankton biomass in this region also affect vertical export (Steinberg et al 2012); we examine here how fluctuations in salp populations (Stone and Steinberg 2014) contribute to vertical carbon flux through a variety of mechanisms.…”

“…bloom in the northeastern Pacific resulted in a major deposition of fecal pellets and carcasses to the seafloor (Smith et al 2014). In addition to producing fecal pellets and carcasses, several abundant species of salps in the Sargasso Sea and elsewhere undergo diel vertical migration, spending time well below the pycnocline during the day and migrating to surface waters at night (Wiebe et al 1979, Madin et al 1996, Stone and Steinberg 2014. While at depth, vertical migrators metabolize particulate organic carbon (POC) consumed in surface waters, respiring it as CO 2 and excreting dissolved organic carbon (DOC), contributing to vertical transport of carbon to depth (Steinberg et al 2001).…”

“…While salps are important contributors to vertical carbon flux while they are present, their populations are quite variable. Salps periodically bloom throughout the world"s oceans, including in the Sargasso Sea (Madin et al 1996, Roman et al 2002, Stone and Steinberg 2014, where they are occasionally the dominant epipelagic zooplankton (Stone and Steinberg 2014). Salps are sensitive to interannual and longer-term changes in the environment, mostly related to variations in temperature and stratification.…”

“…In the Southern Ocean, changes in El Niño-Southern Oscillation (ENSO) and regional warming are correlated with increases in salps (Atkinson et al 2004, Loeb et al 2010, and worldwide, gelatinous zooplankton fluctuations are linked to oscillations in climate indices (Condon et al 2013). In the Sargasso Sea, biomass of the salps Thalia democratica and Cyclosalpa polae increased over the last 20 years, and was positively correlated with water column stratification (Stone and Steinberg 2014). T. democratica abundance was also higher within cyclonic eddies in the Sargasso Sea, possibly through increased eddy-induced production or through eddy-wind aggregation (Stone and Steinberg 2014).…”

An observation of two oceanic salp swarms in the Tasman Sea: Thetys vagina and Cyclosalpa affinis

“…Thetys vagina is often observed opportunistically as a single individual and little is known about their ecology (Nakamura and Yount, 1958;McAlice, 1986;Sims, 1996;Stone and Steinberg, 2014). Large biomasses (900 t WW km −3 ) of T. vagina have been observed in the Japan Sea (Iguchi and Kidokoro, 2006) and Tasman Sea (Thompson, 1948;Henschke et al, 2013).…”