Impacts of aquaculture-derived organic matter sedimentation 

on the trophic characteristics of dominant polychaetes in Sansha Bay

Abstract: Sansha Bay, located in the northeast of Fujian Province, is a typical farming bay in China′s offshore. Aquaculture activities in the bay include cage farming of fish, raft farming of seaweeds, floating farming of abalone, and cage farming of Pseudosciaena crocea. In 2020, the production of P. crocea reached 179 000 tons in Sansha Bay, accounting for more than 70% of the total production of P. crocea in China. Cage farming is a kind of feeding aquaculture, which produces a considerable number of aquaculturederived organic matter (AOM), including waste feed and feces. AOM with higher nitrogen and phosphorus content is released into the environment in both dissolved and particulate form. Dissolved AOM is diluted and diffused by hydrodynamic action, and particulate AOM settles into the sediment. AOM in sediments is ingested by macrobenthos and affects material cycling and energy flow in ecosystems. Polychaetes (Annelida) are a relatively common group of macrobenthos. After settling into the sediment environment, AOM may be ingested by depositfeeding and omnivorous polychaetes, and the nutritional and ecological characteristics of polychaetes will change accordingly. Many studies have focused on the structure of benthic communities in Sansha Bay, and the effects of farming activities on the nutritional and ecological characteristics of macrobenthos have not been reported so far. To understand the benthic ecological and environmental effects of cage farming activities in Sansha Bay, stable carbon and nitrogen isotope ratios (δ13C and δ15N) with numerical model method were used to analyze the impacts of AOM sedimentation on the trophic characteristics of five numerical dominant macrobenthos, Onuphis eremita, Ophelina acuminata, Lumbrineris sp., Heteromastus filiformis and Sternaspis scutata. Potential food sources for polychaetes in Sansha Bay included waste feed, fecal matter, seaweeds (Laminaria japonica and Gracilaria lemaneiformis), particulate organic matter (POM), zooplankton, and benthic microalgae. Five stations were arranged on each section of three sampling transects of different hydrodynamic characteristics. Benthic species and their potential food sources were collected from September 24th to 29th, 2021. The sedimentary organic matter (SOM) samples were also sampled as a mixture of food sources. The δ13C and δ15N of these samples were analyzed.

The results showed that the δ13C and δ15N of potential food sources ranged from -25.51‰ to -12.71‰ and 3.30‰ to 12.91‰, respectively. The δ13C and δ15N of potential food sources showed significant differences, respectively (KruskalWallis test, P<0.001). The δ13C and δ15N of SOM ranged from -25.51‰ to -21.04‰ and 3.79‰ to 7.44‰, respectively. The δ13C and δ15N of SOM showed significant differences, respectively (KruskalWallis test, P<0.05) among three sampling transects. The δ13C and δ15N of POM ranged from -25.08‰ to -22.85‰ and 6.21‰ to 11.47‰, respectively. The δ15N of POM showed significant differences (KruskalWallis test, P<0.01) among three sampling transects. The values of δ13C and δ15N of L. japonica were (-14.61±2.01)‰ and (7.18±2.98)‰, respectively. The values of δ13C and δ15N of G. lemaneiformis were (-18.40±0.27)‰ and (11.60±1.29)‰, respectively. The values of δ13C and δ15N of zooplankton were (-18.17±0.36)‰ and (10.41±0.99)‰, respectively. The values of δ13C and δ15N of benthic microalgae were (-22.63±0.55)‰ and (4.86±1.46)‰, respectively. The values of δ13C and δ15N of waste feed were -(17.58±0.28)‰ and （11.27±0.32）‰, respectively. The values of δ13C and δ15N of fecal matter were （-20.31±1.42）‰ and （7.62±1.47）‰, respectively. The δ13C and δ15N of five dominant polychaetes ranged from -21.82‰ to -16.87‰ and 7.05‰ to 12.98‰, respectively. The δ13C and δ15N of O. eremita ranged from -20.97‰ to -16.87‰ and 9.15‰ to 12.98‰, respectively. The δ13C and δ15N of O. eremita showed significant differences, respectively (KruskalWallis test, P<0.01) among three sampling transects. The δ13C and δ15N of O. acuminata ranged from -18.84‰ to -17.39‰ and 9.76‰ to 10.34‰, respectively. The δ13C and δ15N of Lumbrineris sp. ranged from -19.84‰ to -18.20‰ and 10.36‰ to 12.53‰, respectively. The δ13C and δ15N of H. filiformis ranged from -20.20‰ to -19.20‰ and 10.03‰ to 11.38‰, respectively. The δ13C and δ15N of S. scutata ranged from -21.82‰ to -18.77‰ and 7.05‰ to 8.79‰, respectively. Trophic levels of five species showed significant differences (KruskalWallis test, P<0.001). Omnivorous polychaetes Lumbrineris sp. and O. eremita had the highest trophic levels (2.79 and 2.78, respectively), and depositfeeding polychaetes H. filiformis and O. acuminata had intermediate levels (2.62 and 2.41, respectively), and S. scutata was found at the lowest level (1.81). The δ13C and δ15N of G. lemaneiformis and zooplankton in Sansha Bay were close to those of benthic dominant species (difference<1‰), and the difference of δ13C between L. japonica and benthic dominant species (>4‰) was much higher than the fractionation coefficient of δ13C (about 1‰). G. lemaneiformis, zooplankton, and L. japonica were not the food sources of benthic dominant species in Sansha Bay. Bayesian stable isotope mixing models in R (SIMMR) package were used to estimate the proportional contribution of food sources (waste feed, fecal matter, benthic microalgae, and POM) to the five species. The results showed that AOM was an important food source for dominant polychaetes, contributing （68.4±17.2）%, （61.7±22.0）%, （54.9±13.4）%, （36.4±8.3）%, and （18.3±13.2）% to O. eremita, O. acuminata, Lumbrineris sp., H. filiformis, and S. scutata, respectively. The contribution of fecal matter was （35.0±13.2）%, （53.3±21.7）%, （35.9±16.4）%, （23.4±2.6）%, and （11.0±7.8）% to O. eremita, O. acuminata, Lumbrineris sp., H. filiformis, and S. scutata, respectively. The contribution of waste feed was （33.4±18.4）%, （8.4±7.4）%, （19.0±11.9）%, （13.0±5.6）%, and （7.3±5.4）% to O. eremita, O. acuminata, Lumbrineris sp., H. filiformis, and S. scutata, respectively. Benthic microalgae contributed to the diets of dominant polychaetes as well, contributing （22.5±15.6）%, （33.0±18.7）%, （30.7±15.0）%, （47.6±22.6）%, and （76.6±14.1)% to O. eremita, O. acuminata, Lumbrineris sp., H. filiformis, and S. scutata, respectively. POM contributed to the smallest diets of dominant polychaetes, contributing (9.1±5.1)%, (3.9±1.0)%, (14.4±8.9)%, (16.1±14.3)%, and (5.2±1.3)% to O. eremita, O. acuminata, Lumbrineris sp., H. filiformis, and S. scutata, respectively. In conclusion, the AOM originated from cage farming served as an important food source for macrobenthos in Sansha Bay, based on which it was concluded that the aquaculture activities showed significant effects on benthic ecological environment. The trophic ecological characteristics of macrobenthos in natural waters were affected by combined factors, and the food composition and feeding habits changed the trophic characteristics of macrobenthos in Sansha Bay.

Keywords: Sansha Bay； cage farming； benthic ecological effects； macrobenthos； polychaetes