Ecosystem services transcend boundaries: estuaries provide resource subsidies and influence functional diversity in coastal benthic communities

Abstract

Background: Estuaries are highly productive ecosystems that can export organic matter to coastal seas (the 'outwelling hypothesis'). However the role of this food resource subsidy on coastal ecosystem functioning has not been examined.

Methodology/principal findings: We investigated the influence of estuarine primary production as a resource subsidy and the influence of estuaries on biodiversity and ecosystem functioning in coastal mollusk-dominated sediment communities. Stable isotope values (δ(13)C, δ(15)N) demonstrated that estuarine primary production was exported to the adjacent coast and contributed to secondary production up to 4 km from the estuary mouth. Further, isotope signatures of suspension feeding bivalves on the adjacent coast (Dosinia subrosea) closely mirrored the isotope values of the dominant bivalves inside the estuaries (Austrovenus stutchburyi), indicating utilization of similar organic matter sources. However, the food subsidies varied between estuaries; with estuarine suspended particulate organic matter (SPOM) dominant at Tairua estuary, while seagrass and fringing vegetation detritus was proportionately more important at Whangapoua estuary, with lesser contributions of estuarine SPOM. Distance from the estuary mouth and the size and density of large bivalves (Dosinia spp.) had a significant influence on the composition of biological traits in the coastal macrobenthic communities, signaling the potential influence of these spatial subsidies on ecosystem functioning.

Conclusions/significance: Our study demonstrated that the locations where ecosystem services like productivity are generated are not necessarily where the services are utilized. Further, we identified indirect positive effects of the nutrient subsidies on biodiversity (the estuarine subsidies influenced the bivalves, which in turn affected the diversity and functional trait composition of the coastal sediment macrofaunal communities). These findings highlight the importance of integrative ecosystem-based management that maintains the connectivity of estuarine and coastal ecosystems.

Figure 1. Dual isotope (δ¹³C, δ¹⁵N) plots of estuarine primary producers (crosses) and predicted fractionation associated with one trophic level for Tairua and Whangapoua estuaries.

Dosinia subrosea (foot muscle) from site 1 closest to the estuary mouth are presented as solid circles and from site 9 as empty circles. For comparative purposes, the isotope values of a suspension-feeding bivalve, Austrovenus stutchburyi, within each estuary are presented as grey triangles.

Figure 2. Mixing model outputs showing the range of the potential contribution (0.0 to 1.0 representing 0% to 100%) of different primary producers to venerid bivalves, Austrovenus stutchburyi inside the estuaries and Dosinia subrosea, on the open coast at Tairua (left) and Whangapoua (right) estuaries.

The shaded boxes represent the 75^th (top) to 25^th (bottom) percentiles with the central lines indicating the median. The vertical lines outside the boxes indicate the 95^th (top) and 5^th (bottom) percentiles. MPB = microphytobenthos.

Figure 3. DISTLM ordination results showing the relationship between sites sampled across a gradient from the estuary mouth to ∼4 km offshore and forcing functions for macrobenthic communities (top panel) and biological traits (bottom panel).

D/abundance = number of individuals of D. subrosea; size refers to the mean size of D. subrosea at that site; distance refers to distance from the estuary mouth. Tairua coastal sites are designated as TP1-9 and Whangapoua coastal sites as WM1-9 (Site 1 is closest to the estuary mouth).