Can export of organic matter from estuaries support zooplankton in nearshore, marine plumes?

Abstract

Marine and terrestrial ecosystems are connected via transfers of nutrients and organic matter in river discharges. In coastal seas, such freshwater outflows create prominent turbidity plumes. These plumes are areas of high biological activity in the pelagos, of which zooplankton is a key element. Conceptually, the increased biomass of zooplankton consumers in plumes can be supported by two alternative trophic pathways-consumption of fresh marine phytoplankton production stimulated by riverine nutrients, or direct trophic subsidies through the uptake of terrestrial and estuarine organic matter flushed to sea. The relative importance of these two pathways has not been established previously. Isotopic tracing (carbon and nitrogen) was used to measure the extent of incorporation of marine versus terrestrial matter into mesozooplankton consumers in the plumes off a small estuary in eastern Australia. Replicate zooplankton samples were taken during baseflow conditions with minimal freshwater influence to the sea, and during pulsed discharge events that generated turbidity plumes in coastal waters. Food sources utilized by zooplankton differed among locations and with the strength of freshwater flow. Terrestrial and estuarine carbon only made a sizeable contribution (47%) to the carbon demands of zooplankton in the lower estuary during pulsed freshwater flows. By contrast, in plumes that developed in nearshore marine waters, phytoplankton supplied up to 90% of the dietary carbon of zooplankton feeding in the plumes. Overall, it was ''fresh'' carbon, fixed by marine phytoplankton, the growth of which became stimulated by fluvial nutrient exports, that dominated energy flows in plume regions. The trophic role of terrestrial and estuarine organic exports was comparatively minor. The trophic dynamics of plankton in small coastal plumes is closely linked to variations in freshwater flow, but this coupling operates mainly through the enhancement of in-situ phytoplankton production rather than cross-boundary transfers of organic matter to marine food webs in the pelagos.

Functioning of the planktonic ecosystem on the Gulf of Lions shelf (NW Mediterranean) during spring and its impact on the carbon deposition: a field data and 3-D modelling combined approach

“…As highlighted by the results of our sensitivity analysis (Fig. 14) and in agreement with sedimentological data (Durrieu de Madron et al, 2000), terrestrial organic particles quickly sink on the prodelta area, remaining barely available for zooplankton consumption (Schlacher et al, 2009). Moreover, the phytoplankton growth is mostly inhibited near the river mouth (Naudin et al, 2001).…”

“…has been evidenced between carbon-rich coastal water and Northern Mediterranean Current oligotrophic water at a fixed station off Marseille . On the whole, phytoplankton could provide up to 90 % of the dietary carbon of mesozooplankton feeding in plume regions (Schlacher et al, 2009).…”

Importance of terrestrial subsidies for estuarine food webs in contrasting East African catchments

“…High plant d 15 N are generally associated with hot and dry environments such as in the semi-arid Tana and the grass-dominated Betsiboka catchments (Craine et al 2009, Pardo andNadelhoffer 2010 . This could be because zooplankton feeds selectively, preferring phytoplankton but assimilating both phytoplankton and detritus, depending on availability (Cole et al 2006, Schlacher et al 2009). Since the five estuaries differed greatly in turbidity and relative availability of phytoplankton and terrestrial detritus, the contribution of these two sources to zooplankton differed greatly between sites, and the absence of a relationship between d detritus or algae.…”

Spatial, seasonal, and ontogenetic variations in the significance of detrital pathways and terrestrial carbon for a benthic shark, Chiloscyllium plagiosum (Hemiscylliidae), in a tropical estuary

“…The relative importance of terrestrial carbon is case-specific, varying at multiple temporal and spatial scales, and depending also on the feeding ecology of the species of interest (Heck et al 2008;Connolly et al 2009;). The combined use of SIA, FAP, and FA-specific d 13 C, as used in this study, allows effective diagnosis of the relative importance of allochthonous terrestrial detritus and autochthonous marine-derived detritus carbon pathways in aquatic ecosystems (Lau et al 2009).…”

“…Estuarine habitats are, therefore, highly dynamic and productive and, as such, are ideal habitats for studying the importance of trophic linkages between terrestrial and marine ecosystems (Chanton and Lewis 2002;Connolly et al 2005;Heck et al 2008). The complex dynamics of trophic subsidies in most estuarine habitats are not wellunderstood, especially in situations where strong seasonal variations in climate and river flow alter the magnitude, location and timing of exogenous inputs (Darnaude et al 2004;McLeod and Wing 2009;Schlacher et al 2009). …”