Monsoons and habitat influence trophic pathways and the importance of terrestrial‐marine linkages for estuary sharks

Abstract

Abstract. Tropical estuaries often receive enhanced fluxes of terrestrial derived organic matter and phytoplankton during the wet season, and such monsoonal events may significantly influence the trophic dynamics of these systems. This study examined spatio-temporal terrestrial-marine linkages in a tropical estuary, the Pearl River Estuary (PRE), southern China, by investigating trophic pathways leading to estuary sharks. We investigated spatial (inshore vs. offshore) and seasonal (wet vs. dry season) variation in the relative importance of terrestrial-and marine-derived carbon, so as to assess the contribution of detrital pathways to the pelagic spadenose shark, Scoliodon laticaudus; ontogentic changes in shark diets were also documented. Stable isotope analyses (SIA) and fatty acid (FA) profiling indicated that spadenose sharks assimilated both marine and terrestrial carbon via consumption of zooplantivorous fish and shrimps. Detrital carbon sources were more important to juvenile and pre-mature sharks at inshore locations, especially during the wet season when river discharge increased and terrestrial detritus was more abundant. Ontogenetic dietary shifts were evident: juvenile and pre-mature sharks had significantly higher levels of bacterial (detrital) FA than adults which contained more animal-derived FA. Inshore sharks, with more depleted d 13 C signatures, relied more on terrestrial carbon than sharks offshore. Comparison of spadenose shark FA profiles with those of the sympatric, white-spotted bamboo shark (Chiloscyllium plagiosum)-a benthic predator that acquires detrital carbon via consumption of polychaetes and crustaceans-revealed that they made greater use of detrital carbon sources. However, spadenose sharks in the inner estuary assimilated higher proportions of terrestrial detritus (44-56%) than bamboo sharks (31-45%). The importance of terrestrial detritus for both shark species demonstrated the important contribution of terrestrial detritus to both pelagic and benthic food webs in the PRE. Terrestrial-marine linkages are therefore of great significance, particularly during the wet season, in this estuarine system, which serves as feeding and nursery grounds for both shark species, and trophic subsidies from land are likely to be important for marine predators in other tropical estuaries.

Spatio-Temporal Occurrence Patterns of Young Sharks in Tropical Coastal Waters

“…The temporal distribution of neonates corroborates the general predominance of summer parturition in the study species across tropical Australia (Last and Stevens 2009). During summer, neonate and YOY sharks may benefit from enhanced energetic uptake in warmer temperatures (Bernal et al 2012) and seasonal pulses in ecosystem productivity (Kneebone et al 2012;Matich and Heithaus 2014;Wai et al 2012). However, neonate pigeye sharks occurred slightly later than expected based on the published months of parturition in Australian waters (around November or December; Last and Stevens 2009).…”

“…Seasonal temperature fluctuations along sub-tropical-temperate coastlines can necessitate marked seasonality in both parturition and nursery utilisation, including large-scale seasonal migrations by young sharks between climatic zones McCandless et al 2007). Sub-tropical-tropical waters experience comparably small temperature changes; however, there are seasonal fluctuations in other variables including rainfall ) and prey availability (Staples 1979;Wai et al 2012). Accordingly, shark species occurring in sub-tropical-tropical waters exhibit a range of reproductive cycles from aseasonal (Harry et al 2010) to seasonal (Stevens and Lyle 1989), and species composition often changes throughout the year (Simpfendorfer and Milward 1993;Taylor and Bennett 2013).…”

“…For example, sharks born during warmer months may benefit from enhanced energetic uptake (Bernal et al 2012) or seasonal pulses in ecosystem productivity and food resources (Kneebone et al 2012;Matich and Heithaus 2014;Wai et al 2012). Temporal dynamics may also influence vulnerability to natural and anthropogenic impacts.…”

Resource partitioning between two young‐of‐year cownose rays Rhinoptera bonasus and R. brasiliensis within a communal nursery inferred by trophic biomarkers

“…When considering only FA profiles, liver tissue showed the strongest difference between species, with up to 18% less PUFA in liver of R. brasiliensis when compared with R. bonasus . Despite this, R. brasiliensis did not show an essential FA deficiency in liver and muscle tissue as previously reported for young placental sharks at birth ( i.e ., DHA and ARA < 6%; Belicka et al, ; Wai et al, ). Considering that liver tissue was principally analysed for YOY < 50 cm W D R. brasiliensis (< 4 months old, Fisher et al, ; Rangel et al, ) and it has a relatively fast turnover rate (Figure ; c .…”

Combined effects of temperature, salinity, and diet simulating upwelling and nonupwelling seasons alter life‐history characteristics of a tropical invertebrate

“…In temperate systems where reproduction and biological productivity are largely linked to the summer, the impacts of such changes may involve a lengthening of the growing season as well as changes in the timing of spring reproductive events (Sparks & Menzel, ). In tropical systems switches between monsoonal and nonmonsoonal conditions, which are driven by changes in the position of the intertropical convergence zone (ITCZ), may dominate the seasonal cycles of many organisms (e.g., Gaonkar et al, ; Pillai & Subramoniam, ; Wai et al, ). Such patterns are already impacted by El Niño‐Southern Oscillation (ENSO; the cycle of sea surface temperatures in the central and eastern Pacific, which has global climate consequences through shifts in atmospheric circulation; see Glynn, ) events which, for example, increase the length of the dry season in Panama.…”