Distribution of life-history stages of the salp Thalia democratica in shelf waters during a spring bloom

Abstract

Swarms of the salp Thalia democratica periodically occur off southeast Australia following the austral spring bloom of phytoplankton. The present study aimed to determine the abundance and size/stage distribution of T. democratica and their relationship with copepods in 3 water types of the western Tasman Sea. Samples were taken from vertical net hauls along 4 cross-shelf transects spaced along 200 km of the New South Wales coast, from the East Australian Current (EAC) separation zone, around 32.5°S, to off Sydney (34°S). Temperature-salinity signatures grouped stations into 3 distinct water types: inner shelf water, EAC and upwelled water. Although common across all stations, T. democratica was significantly more abundant in inner shelf waters compared to both EAC and upwelled water. Analysis of population structure (aggregate buds, aggregate females, aggregate males, immature solitaries and mature solitaries) also identified higher proportions of reproductive aggregates and their offspring in inner shelf water. This salp population structure was significantly different in the EAC regions, characterised by a paucity of the solitary stages, higher temperatures and lower chlorophyll a concentrations. A weak negative correlation was identified between T. democratica and copepod abundance. In the present study, the maximum abundance of T. democratica was twice the highest globally recorded abundance and 10-fold greater than maximum abundances sampled from the continental shelf and slope waters off southeast Australia during the period from 1938 to 1942. KEY WORDS: Thalia democratica · Salps · Life cycle · Zooplankton · Water types · Population structure · East Australian CurrentResale or republication not permitted without written consent of the publisher Mediterranean (Licandro et al. 2006) and South Africa (Gibbons 1997). Occasional swarms of gelatinous species, including salps, are well known historically, but recent reports indicate that these are increasing in frequency and magnitude as a result of human-induced stresses such as eutrophication and climate change (Hay 2006).Salp swarms are possible as a result of their life cycle involving the obligatory alternation between aggregated sexual and solitary asexual generations (Fig. 1), allowing Thalia democratica populations to grow exponentially while maintaining genetic variability (Godeaux et al. 1998). The aggregate stage begins life as a chain of genetically identical individuals ('buds' or A1) and reproduces sexually. Born female, aggregates separate and are externally fertilised immediately after release from their parent, and an embryo grows internally (A2). Once the embryo is born, female aggregates develop testes and function as a male (A3). These males (A3) then fertilise the recently released buds (A1) before dying shortly thereafter. The embryo is the start of the solitary generation (S1) that asexually produces up to 3 chains, each with between 20 and 80 individual aggregates (A1). Once mature (S2), solitaries release each chain separatel...

Three-dimensional structure of a swarm of the salpThalia democraticawithin a cold-core eddy off southeast Australia

“…The processes leading to the formation of the eddy north of Seal Rocks (Figure 2) were investigated using temperature and salinity properties from the same cruise detailed by Henschke et al [2011]. In the vicinity of the formation, inner shelf water (mean properties in the top 50 m; T = 19°C, S = 35.55, σ = 1025.45 kg m −3 ) and upwelled water (T = 17.2°C, S = 35.42, σ = 1025.80 kg m −3 ) were present [ Henschke et al , 2011]. At the center of the CCE during the sampling period, the SeaSoar transects (Figures 8 and 9) show the inner shelf water above 20 m (19°C) and upwelled water below 40 m (17°C).…”

“…Previous studies have shown OPC observations of gelatinous organisms to be problematic [ Remsen et al , 2004; Gonzalez‐Quiros and Checkley , 2006]. In order to assess the OPC for this application, we compare the in situ abundance and size distribution of samples from the EZ nets (this study) and N70 nets [ Henschke et al , 2011] with OPC determined values. The N70 net is a 70 cm diameter net which was used extensively during the 1938 to 1942 M/V Warreen cruises to sample zooplankton off southeast Australia [ Thompson and Kesteven , 1942].…”

Zooplankton trophic niches respond to different water types of the western Tasman Sea: A stable isotope analysis

“…Despite the larger niche width, the lack of niche differentiation between E. elongatus and the other two species suggests that E. elongatus may compete with both T. gregaria and T. democratica for food (e.g. Henschke et al, 2011). However, as niche overlap does not necessarily confirm that organisms are competing for the same food source, instead that they could be consuming food sources with similar isotopic signatures, an investigation of diet composition and foraging behaviour (such as spatial segregation) is needed to reveal the degree of competition that is occurring between these zooplankton species.…”

“…The δ 13 C and δ 15 N stable isotope signature of POM has been found to vary across water types in relation to different oceanographic conditions (Stowasser et al, 2012). Off southeast Australia, the western Tasman Sea has different water types including cyclonic cold core eddies (CCE), anti-cyclonic warm core (WCE) eddies (Nilsson and Cresswell, 1981) and nutrient-rich inner shelf (IS) waters (Henschke et al, 2011). CCEs may entrain IS water (Mullaney and Suthers, 2013), resulting in a 16% increase in chlorophyll a compared to surrounding waters (Everett et al, 2012).…”

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

“…Due to their regular occurrence (Henschke et al, 2014) and coastal dominance (Henschke et al 2011), smaller salps such as Thalia democratica are studied with greater frequency. Although these smaller salps contribute significantly to biogeochemical cycling with fast growth rates (Heron, 1972a) and faecal pellets, carcasses of these smaller salps rarely sink as they are neutrally buoyant (Tsukamoto et al, 2009).…”