Land–Ocean Coupling of Carbon and Nitrogen Fluxes on Sandy Beaches

Abstract

Rivers link oceans with the land, creating global hot spots of carbon processing in coastal seas. Coastlines around the world are dominated by sandy beaches, but beaches are unusual in that they are thought to rely almost exclusively on marine imports for food. No significant connections to terrestrial production having been demonstrated. By contrast, we isotopically traced carbon and nitrogen pathways leading to clams (Donax deltoides) on beaches. Clams from areas influenced by river plumes had significantly different isotope signatures (δ 13 C:-18.5-20.2 ‰; δ 15 N: 8.3-10.0 ‰) to clams remote from plumes (δ 13 C: 17.5-19.5 ‰; δ 15 N: 7.6-8.7 ‰), showing that that terrestrial carbon and sewage, both delivered in river plumes, penetrate beach food webs. This is a novel mechanism of trophic subsidy in marine intertidal systems, linking the world's largest shore ecosystem to continental watersheds. The same clams also carry pollution signatures of sewage discharged into rivers, demonstrating that coastal rivers connect ecosystems in unexpected ways and transfer contaminants across the land-ocean boundary. The links we demonstrate between terrigenous matter and the largest of all marine intertidal ecosystems are significant given the immense social, cultural and economic values of beaches to humans and the predicted consequences of altered river discharge to coastal seas caused by global climate change.

Terrestrial trophic subsidy in sandy beaches: evidence from stable isotope analysis in organic matter sources and isopod Excirolana armata

“…We have also shown that TOM was consumed by the isopod at Coronilla, closer to the canal discharge, even though the sporadic input usually observed at this site precluded the detection of this food source during sampling. This is in agreement with previous findings on small-plume discharges, which have highlighted the fact that brief allochthonous inputs to nearshore marine waters have a limited zone of influence (Gaston et al 2006, Connolly et al 2009). It should be noted that TOM could also be indirectly assimilated by E. armata via POM, since fragmentation and physical transport of TOM can account for a large proportion of the organic component of seawater POM (Polis et al 1997).…”

“…In summary, our study reinforces the notion that terrestrial runoff and small plumes constitute important linkages between marine and terrestrial ecosystems (Darnaude et al 2004, Connolly et al 2009), but that their zone of influence may be limited (Gaston et al 2006). This link increases food sources in marine ecosystems, but may also lead to transport of contaminated sediments to sea, even forming a contaminant 'imprint' on the seafloor (Dagg et al 2004, Gaston et al 2006.…”

“…Similar isotopic depletion in coastal benthic invertebrates has been recognized as caused by the assimilation of TOM originating from freshwater discharges (Riera & Richard 1997, Darnaude at al. 2004, Schlacher & Connolly 2009). Particularly for sandybeach ecosystems, we have demonstrated the important role played by TOM coming from a freshwater discharge in the food diet of the most conspicuous inhabitant of SW Atlantic sandy shores, viz.…”

“…On sandy beaches, food webs are based mainly on marine sources, such as phytoplankton, wrack (stranded algae and seagrasses) and carrion (McLachlan & Brown 2006), with the influence of these marine subsidies decreasing at increasing distances inland (Colombini et al 2011). However, little is known about the consequences to these ecosystems of terrestrial-resource subsidies (but see Schlacher & Connolly 2009, Colombini et al 2011. The links between terrigenous matter and the marine intertidal system are much closer than previously estimated (Schlacher & Connolly 2009).…”

“…However, little is known about the consequences to these ecosystems of terrestrial-resource subsidies (but see Schlacher & Connolly 2009, Colombini et al 2011. The links between terrigenous matter and the marine intertidal system are much closer than previously estimated (Schlacher & Connolly 2009). Moreover, recent research has demonstrated that impacts of river inputs on marine coastal ecosystem dynamics include increases in terrestrial particulate organic matter (POM), which is directly consumed by depositfeeding invertebrates, and which also affects fish abundance (Darnaude et al 2004, Sampaio et al 2010.…”