The effects of seaweed and shellfish mariculture on the fecal carbon flux of zooplankton in a mariculture base of southern China: Implications for the biological pump

Abstract

Zooplankton play a pivotal role in the marine ecosystem, with their fecal pellets constituting the primary component of passively sinking particles in the ocean. The sinking of fecal pellets provides an effective vector for the transfer and sequestration of particulate organic carbon in the sea. Mariculture activities are extensively practiced along the coastline of China. We hypothesized that seaweed and shellfish mariculture would differentially affect zooplankton community structure and fecal carbon flux due to differences in habitat provision and food source. Aimed to test this hypothesis, surveys were carried out in different mariculture areas around Nan'ao Island in the South China Sea from March to June 2023 during seaweed Gracilaris lemaneiformis cultivation. A total of 36 zooplankton species were identified, including 27 copepod species. Zooplankton abundance ranged from 500 to 2760 ind. m⁻³, with the highest abundance observed in the G.lemaneiformis cultivation area (P < 0.05). Furthermore, the Shannon-Wiener diversity index of the zooplankton community in the G.lemaneiformis area surpassed that of the shellfish and control areas (P < 0.05). The abundance of zooplankton fecal pellets fluctuated between 12 and 283 pellets m⁻³, and the fecal pellet carbon flux ranged from 0.004 to 0.976 mg C m⁻² d⁻¹. The fecal pellet abundance and carbon flux of zooplankton peaked in the G.lemaneiformis cultivation area (P < 0.05). Results showed mariculture activities affect zooplankton community structure and their fecal pellet carbon flux; the large-scale cultivation of G.lemaneiformis enhances zooplankton diversity and effectively increases the carbon flux of fecal pellets in the coastal waters. These findings could provide new management strategies for mariculture environments with increasing anthropogenic stressors in the Anthropocene Epoch.