Major Sources of Organic Matter in a Complex Coral Reef Lagoon: Identification from Isotopic Signatures (δ13C and δ15N)

Abstract

A wide investigation was conducted into the main organic matter (OM) sources supporting coral reef trophic networks in the lagoon of New Caledonia. Sampling included different reef locations (fringing, intermediate and barrier reef), different associated ecosystems (mangroves and seagrass beds) and rivers. In total, 30 taxa of macrophytes, plus pools of particulate and sedimentary OM (POM and SOM) were sampled. Isotopic signatures (C and N) of each OM sources was characterized and the composition of OM pools assessed. In addition, spatial and seasonal variations of reef OM sources were examined. Mangroves isotopic signatures were the most C-depleted (-30.17 ± 0.41 ‰) and seagrass signatures were the most C-enriched (-4.36 ± 0.72 ‰). Trichodesmium spp. had the most N-depleted signatures (-0.14 ± 0.03 ‰) whereas mangroves had the most N-enriched signatures (6.47 ± 0.41 ‰). The composition of POM and SOM varied along a coast-to-barrier reef gradient. River POM and marine POM contributed equally to coastal POM, whereas marine POM represented 90% of the POM on barrier reefs, compared to 10% river POM. The relative importance of river POM, marine POM and mangroves to the SOM pool decreased from fringing to barrier reefs. Conversely, the relative importance of seagrass, Trichodesmium spp. and macroalgae increased along this gradient. Overall, spatial fluctuations in POM and SOM were much greater than in primary producers. Seasonal fluctuations were low for all OM sources. Our results demonstrated that a large variety of OM sources sustain coral reefs, varying in their origin, composition and role and suggest that δ13C was a more useful fingerprint than δ15N in this endeavour. This study also suggested substantial OM exchanges and trophic connections between coral reefs and surrounding ecosystems. Finally, the importance of accounting for environmental characteristics at small temporal and spatial scales before drawing general patterns is highlighted.

Evaluation of anthropogenic influences on the Luhuitou fringing reef via spatial and temporal analyses (from isotopic values)

“…The mean δ 15 N value of the POM was +7.04 ± 3.81‰ (Figure and Table ). On the basis of published isotopic values from various sources, the mean value suggests that the nitrogen was derived from terrestrial sewage (+3 to +8‰) [ Higueras et al ., ; Khodse and Bhosle , ; Lamb et al ., ; Rogers , ], marine upwelling (+4 to +7‰) [ Bode , ; Briand et al ., ; Lamb et al ., ] and animal waste (>10‰) [ Lindau et al ., ; Sánchez et al ., ; Swart et al ., ]. Atmospheric dinitrogen is 0‰ [ Mariotti , ], terrestrial plant (vascular plant) nitrogen has a δ 15 N value of −2 to +2‰ [ Higueras et al ., ; Kendall et al ., ; Lin et al ., ], and nitrogen fixation and assimilation have δ 15 N values of 0 to +1‰ [ Kaiser et al ., ; Lamb‐Wozniak , ] and +1 to +3‰ [ Burford et al, ; Higueras et al ., ], respectively.…”

“…Therefore, pollution from more extensive region and anthropogenic activities from riverine sewage discharges adjacent to reefs should be focused to effectively reduce human-derived nutrients on reefs.Numerous studies have shown that natural stable isotopes (d 13 C and d 15 N) of particulate organic matter (POM) are good nutrient source indicators, as phytoplankton and other microorganisms present in the water column take up and turn over nutrients in a relatively short period of time (hours to weeks) [Loh and Bauer, 2000]. Stable isotopes of POM can be used to investigate the origin, fate, and seasonal variation in nutrients in aquatic environments [Briand et al, 2015;Cao et al, 2015]. Marine POM comprises a spectrum of colloidal and particulate matter from terrestrial detritus, riverine particulate matter, marine tidally Key Points:Isotopic values revealed strong anthropogenic influences on coral reefs Hydrodynamic conditions coupled with human activities have profound implications for nutrient concentrations There might be more inputs from terrestrial detritus and anthropogenic-derived discharges in dry seasons…”

Understanding the fate of shrimp aquaculture effluent in a mangrove ecosystem: Aiding management for coastal conservation

“…In the KKB marine system, terrestrial inputs to SOM generated marked depletions in SOM δ 13 C values ( M ± SD : –24.5 ± 0.6‰) compared to other Pacific bays without aquaculture, including those in Southwest Thailand (–21.0 ± 2.0‰; Kuramoto & Minagawa, ), Korea (–20.6 ± 0.9‰; Kang et al, ) and New Caledonia (–17.9 ± 3.6‰; Briand et al, ). These more enriched δ 13 C SOM values indicate greater contributions to marine SOM from marine benthic primary producers (seagrass, algal turf, macroalgae).…”

“…A 13 C‐enrichment in mangrove sediments relative to mangrove plant materials is either from the mangrove signature being diluted through mixing with material enriched in 13 C or there is a substantial change in isotopic composition associated with OM degradation during early diagenesis (Muzuka & Shunula, ). The first case can occur due to the presence of (epiphytic) algae in mangrove SOM, as a compilation of ẟ 13 C values for mangrove benthic algae, including epiphytic and edaphic algae, shows that algae are generally enriched relative to mangroves leaves (Bouillon et al, ; Briand et al, ). Bacteria and fungi cause the second scenario: decomposition of mangrove leaves, seagrasses and other plant materials enrich 13 C by up to 2‰ (Fourqurean & Schrlau, ; Lehman, Bernasconi, Barbieri, & McKenzie, ).…”

“…Carbon stable isotopes (δ 13 C) provide a means to discriminate autochthonous (internal) and allochthonous (external) OM input to mangrove and marine ecosystems (e.g. Briand, Bonnet, Goiran, Guillou, & Letourneur, ). The δ 13 C values of different primary producers (i.e.…”