Changes in Water Quality and Plankton of Artificial Culture Pond in Sea Cucumber Apostichopus japonicus

Recently, how to improve the aquaculture efficiency of sea cucumber Apostichopus japonicus and promote the sustainable development of its artificial cultivation has become an increasingly important issue. The pond water environment plays important roles in the survival rate and growth of A. japonicus seedlings. This study investigated the changes in water quality and plankton from June to November in A. japonicus ponds. The seawater temperature, pH, dissolved oxygen, salinity, nitrogen, and active phosphate contents were measured, and the planktonic species were detected and identified. The results showed that the seawater temperature ranged from 11.2 to 29.9 °C, and the highest temperature did not exceed the tolerance survival limits of A. japonicus. The changes in pH, dissolved oxygen, and salinity were also suitable for growth. A total of six phyla and 14 species of planktonic algae were detected, among which diatoms were dominant, and the dominant species changed over time. In the early stage, it was Chroomonas acuta, then, after it was Nitzschia sp., and then it returned to C. acuta again later. The biomass and density of algae peaked in week 5 (p < 0.05), but decreased to their lowest in week 18. The changes in chlorophyll-a content were consistent with the biomass of algae. Both the chlorophyll-a and pheophytin contents peaked in weeks 5 and 10 (p < 0.05). The changes in suspended particulate matter (SPM) and particulate organic matter (POM) were synchronized, and they peaked in weeks 5 and 12. These results suggested that the planktonic algae have the functions of a food supply and an environmental indication, and changes in chlorophyll-a, pheophytin, SPM, and POM support the food source reserve for A. japonicus. This study provides important information for the artificial cultivation of sea cucumber seedlings in a pond, and it is useful to promote the sustainable development of the sea cucumber industry.