Novel Application of a Compound-Specific Stable Isotope (CSSI) Tracking Technique Demonstrates Connectivity Between Terrestrial and Deep-Sea Ecosystems via Submarine Canyons

Abstract

Studies have shown the importance of submarine canyons as conduits of land-derived organic carbon beyond the coastal shelf into the deep-sea where a single obvious river source can be identified. When there is more than one river source, identifying which rivers contribute to canyon sediment organic matter is technically challenging. Here, we compare two contrasting submarine canyons: the Hokitika Canyon, a long, narrow, and gently sloping canyon on the west coast of New Zealand; and the Kaikōura Canyon, a high productivity, short, steep canyon close to shore on the east coast of New Zealand. Both canyons have multiple potential river sources, so we applied a compound specific stable isotope (CSSI) tracking technique to identify and apportion the contribution from each river at locations along the length of each canyon axis. We found that land-derived organic matter contributed between 74 and 100% of the total organic matter in the sediment of the Hokitika Canyon as far as 200 km from shore and to depths of 2000 m. However, less than 50% of the land-derived organic carbon came from the largest river closest to the canyon head. We hypothesize that longshore drift transported much of the sediment from that river past the Hokitika Canyon, while river inflows farther up-current supplied the bulk of the land-derived organic carbon. In contrast, land-derived organic matter contributed less than 50% of the total organic matter in Kaikōura Canyon sediments with land-derived organic sediment contribution decreasing steeply to less than 15% at about 24 km from shore in 1500 m water depth. Most of the land-derived organic matter (ca. 80%) came from the river with the largest suspended sediment yield, despite another (smaller) river discharging closer to the canyon head. We hypothesize that this difference in carbon source is partly due to

Structure of infaunal communities in New Zealand submarine canyons is linked to origins of sediment organic matter

“…Relationships between environmental predictor variables and meiofaunal and macrofaunal community parameters (abundance, biomass, community structure) were investigated using distance‐based linear models (DistLMs) in PERMANOVA+ (Anderson et al ). In addition to the sediment parameters described above, data from Gibbs et al (under review) on the relative proportions of marine‐ and land‐derived organic carbon at the canyon sampling sites derived from bulk carbon stable isotopic mixing models were used to investigate potential relationships with infaunal parameters. Gibbs et al (under review) determined the proportional contribution of land‐ and marine‐derived organic matter by comparing it to offshore (100% marine‐derived organic matter) and riverbed (100% land‐derived organic matter) sediments using a simple IsoError mixing model based on bulk stable isotope values of carbon (δ13C) (Phillips and Gregg ).…”

“…In addition to the sediment parameters described above, data from Gibbs et al (under review) on the relative proportions of marine‐ and land‐derived organic carbon at the canyon sampling sites derived from bulk carbon stable isotopic mixing models were used to investigate potential relationships with infaunal parameters. Gibbs et al (under review) determined the proportional contribution of land‐ and marine‐derived organic matter by comparing it to offshore (100% marine‐derived organic matter) and riverbed (100% land‐derived organic matter) sediments using a simple IsoError mixing model based on bulk stable isotope values of carbon (δ13C) (Phillips and Gregg ). Data from Gibbs et al (under review) were available from six of the Kaikōura Canyon sampling sites (TAN1006, see Table ), and six of the Hokitika Canyon sampling sites (TAN1311).…”

“…The two study areas are described in detail in Gibbs et al (under review). For completeness, the study area descriptions are repeated here.…”

“…The present study contributes to an overall project that seeks to understand the variable influence of land‐derived organic matter on deep‐sea benthic communities, and how these relationships may be mediated by the physical characteristics of canyons. A companion study examined the quantity, quality, and sources of the sediment organic matter (Gibbs et al under review), whereas the aim of the present study (using largely existing data and samples) was to investigate the relationships between sediment organic matter parameters and macrofaunal and meiofaunal abundance, biomass, and community structure of two contrasting canyons. The two study canyons were Kaikōura Canyon, which has a steep bathymetric gradient and is close to the east coast of South Island/Te Waipounamu of New Zealand/Aotearoa, and Hokitika Canyon, which is connected to the South Island west coast by a longer, narrow channel‐like feature with a lower gradient.…”

“…The two study canyons were Kaikōura Canyon, which has a steep bathymetric gradient and is close to the east coast of South Island/Te Waipounamu of New Zealand/Aotearoa, and Hokitika Canyon, which is connected to the South Island west coast by a longer, narrow channel‐like feature with a lower gradient. The overall project aimed to test the following hypotheses: (1) that the steep topography and proximity to the coast of Kaikōura Canyon results in higher organic matter enrichment from land sources relative to Hokitika Canyon (Gibbs et al under review), and (2) that within‐ and between‐canyon variability in organic matter availability from land and marine sources is an important driver of infauna community parameters. Here, we test the second hypothesis by investigating relationships between environmental parameters (including concentrations of land‐ and marine‐derived organic matter) and infaunal community parameters, including preliminary stable isotope analyses of benthic invertebrates from Kaikōura Canyon.…”