Copepods are the most abundant and diverse mesozooplankton in marine habitats and are considered to make a major contribution to secondary production in the marine food web. Copepods represent important trophic linkages between the primary producers, phytoplankton, and higher trophic levels such as fish. In coastal waters, copepod community structure often fluctuates with seasonal changes in water temperature, salinity and phytoplankton abundance. Community structure can also be greatly affected by human activities in or near ecosystems such as estuaries and bays, which are often polluted by domestic and industrial wastewaters. Because of this, the community structure and diversity of copepods could be used as a bioindicator of current condition or health of the coastal ecosystem.
Zhanjiang Bay is an economically and militarily important subtropical harbor located in the northeast of the Leizhou Peninsula, Guangdong, China. It is near Naozhou Island, Donghai Island and Nansan Island, and covers 264.9 km². The rapid development of industry and increase in the human population in this region over recent years, has led to increasing levels of pollution in the bay. To gain a basic understanding of the current state of the bay ecosystem, we investigated temporal and spatial changes of the zooplankton community in Zhanjiang Bay.
Plankton samples were collected from 28 stations in February (winter), May (spring), August (summer) and November (fall) of 2009. These stations included sites closed to aquaculture areas (Stations 4, 12, 15, 22), near docks (14, 17, 18, 26, 27) and in the waterways (e.g., 2, 9, 13, 16, 19, 21, 25). Zooplankton samples were collected using a vertical net tow and plankton net (Shallow Water Type I, equipped with Digital Flow Meter), and then fixed in 5% formalin to be brought back to the laboratory for microscopic analysis. Phytoplankton abundances were estimated using a concentrated cell count method. Measurements were also made of chlorophyll a, dissolved inorganic nitrogen, phosphate and silicate concentrations.
Our primary focus was to investigate any seasonal changes of the planktonic copepod community. In total, 72 species of copepods were identified, with 36, 33, 24, and 19 found in winter, spring, summer and fall, respectively. This represented 29.5%, 39.9%, 29.6% and 40.4% of the total number of zooplankton species found in the samples in each of these seasons, respectively. The copepods could be divided into four ecological groups: estuarine/brackish water, coastal warm temperate water, coastal warm water and open sea warm water. The coastal warm water group was the dominant group in terms of number of species and individuals, which is typical of a subtropical coastal copepod community. The dominant species were Calanus sinicus, Paracalanus crassirostris, Oithona brevicornis, Eucalanus subcrassus, Acartia spinicauda and some unidentified copepods. The genus with the most copepod species was Oithona (9 species), followed by Centropages (6), Labidocera (5) and Acartia (4), Microsetella (4), Paracalanus (4) and Pontellopsis (4). Copepod species that were common to any two seasons ranged from 6-14 species, with a seasonal change rate of 0.57-0.88. This rate was highest when comparing the fall/winter species and lowest for the summer/fall species. Only four species and one copepodite were found in all seasons, while eight were common to three seasons. This indicates that there were significant changes in the copepod community between seasons. Total copepod abundance was highest in the summer (960.0 individuals m^-3), followed by spring (421.0), winter (303.4) and fall (252.8). Copepod abundance was higher in the inner bay, and lower in the outer bay, during the spring and winter; while this trend was reversed in the summer. In the fall, the distribution of copepods was relatively uniform throughout the bay.
There was a significant positive correlation between copepod abundance and water temperature, phytoplankton abundance and chlorophyll α concentration. There was a negative correlation between copepod abundance and the concentrations of dissolved inorganic nitrogen and phosphate. There was no evidence of effects of salinity, pH or silicate concentration on copepod abundance. Compared to other regions of inside and outside of China that have been studied, the copepod community in Zhanjiang Bay has higher biodiversity and a more uniform distribution, which could indicate that the bay ecosystem is currently in a relatively stable and healthy condition.