Millennial‐aged organic carbon subsidies to a modern river food web

Abstract

Recent studies indicate that highly aged material is a major component of organic matter transported by most rivers. However, few studies have used natural 14C to trace the potential entry of this aged material into modern river food webs. Here we use natural abundance 14C, 13C, and deuterium (2H) to trace the contribution of aged and contemporary organic matter to an important group of consumers, crustacean zooplankton, in a large temperate river (the Hudson River, New York, USA). Zooplankton were highly 14C depleted (mean delta14C = -240 per thousand) compared to modern primary production in the river or its watershed (delta14C = -60 per thousand to +50 per thousand). In order to account for the observed 14C depletion, zooplankton must be subsidized by highly aged particulate organic carbon. IsoSource modeling suggests that the range of the aged dietary subsidy is between approximately 57%, if the aged organic matter source was produced 3400 years ago, and approximately 21%, if the organic carbon used is > or = 50 000 years in age, including fossil material that is millions of years in age. The magnitude of this aged carbon subsidy to river zooplankton suggests that modern river food webs may in some cases be buffered from the limitations set by present-day primary production.

Strong evidence for terrestrial support of zooplankton in small lakes based on stable isotopes of carbon, nitrogen, and hydrogen

“…1 and Table S3). Dilution-regrowth cultures of phytoplankton (Methods) from these lakes confirmed the large fractionation for δ 2 H between water and algae reported (34,39), and there was little difference between deep and surface water phytoplankton. In the surface waters of both lakes, phytoplankton had a δ 2 H of near −200‰ with low variance (Table S3).…”

“…One intriguing study used specific fatty acid biomarkers and traced the C supporting whitefish (Coregonus 11) to terrestrial material that transferred to fish via copepods. Another study that used ambient 14 C, which can help distinguish materials based on their age found high allochthony for zooplankton, in the Hudson River where old allochthonous carbon is important (34).…”

Use of Multi-Carbon Sources by Zooplankton in an Oligotrophic Lake in the Tibetan Plateau

“…Radiocarbon 14 C isotope measurements were performed by the Accelerator Mass Spectrometry Facility at the Beta Analytic Radiocarbon Dating Laboratory, which is an ISO 17025-accredited radiocarbon dating laboratory in Miami, Florida. The ∆ 14 C value is expressed as ∆ 14 C (‰) = δ 14 C − 2(δ 13 C + 25) (1 + δ 14 C/1000), i.e., the deviation in parts per thousand (‰) from the 14 C activity of 1950s oxalic acid, where δ 14 C (‰) = [(Rsample/Rstandard) − 1] × 10 3 , R = 14 C/ 12 C. The analytical precision for 14 C analyses averaged ±3.73‰ [25].…”

“…This isotope-mixing model solves iteratively for possible carbon source composition and provides a frequency distribution of the contribution of each carbon source to the consumer [28]. We used tolerance parameters of 1‰-3‰, reflecting the high uncertainty of the sources, no trophic enrichment factor for ∆ 14 C (because of its definition), and a trophic adjustment of 0.4‰ for δ 13 C [24,25]. The ∆ 14 C of DIC was used as a surrogate for the ∆ 14 C of phytoplankton since algae biomass production is based on in situ DIC and therefore tracks that of DIC [29][30][31].…”

Freshwater in flux

“…Some of the key members of aquatic food webs, from zooplankton to benthic invertebrates and fish, bear the isotopic or fatty-acid signatures of terrestrial organic matter, indicative of at least some degree of terrestrial support. Why and how does organic matter that has been stable on land for years to centuries become available to the food web once it enters aquatic systems 5 ? The idea that aquatic food webs are supported by terrestrial sources is controversial.…”