Seasonal Changes in the Plankton Community in the Petrochemical Salt Ponds of Mahshahr and Odleh Creek

Introduction: Phytoplankton are the primary producers at the base of food webs in aquatic ecosystems. Marine zooplankton are the secondary producers of the marine food web. The distribution of zooplankton affects the growth potential of fish in the area, especially their larvae. The sensitivity of plankton to slight changes in the environment has made them suitable biological indicators that reflect the health conditions of the aquatic ecosystem. In the Mahshahr salt ponds, during the dewatering stage, shrimp and fish larvae are introduced into the ponds through pumping from Odleh Creek, which is the only source of water supply to the ponds, where they grow in harsh conditions. As a result, regular assessment is necessary to protect the inhabitants of this aquatic ecosystem. The purpose of this study was to investigate the biological health of the Mahshahr salt ponds and Odleh Creek ecosystem using the Water Quality Score (WQS) and ecological indicators based on the planktonic communities of this ecosystem.Materials and Methods: Sampling was conducted at seven stations, from Odleh Creek to Salt Ponds 1 and 2. Water samples for the investigation of phytoplankton and zooplankton were collected using a plankton net across four seasons, from May 2023 to April 2024. Physical and chemical factors, including dissolved oxygen, pH, total phosphate, total nitrogen, and biochemical oxygen demand, were measured to evaluate the WQS index. Some physicochemical factors such as pH, temperature, and dissolved oxygen were measured at the sampling site using a portable multi-parameter device, and salinity was measured using a salinometer. The remaining parameters were measured using standard experimental methods.Results: During this research, moving away from the estuary and towards the ponds showed an increasing trend in the abundance of phytoplankton, salinity, nitrate and dominance index, while the abundance of zooplankton, phosphate, diversity index, species richness, and biological balance exhibited a decreasing trend. A winter peak was observed for phytoplankton density, followed by a spring peak for zooplankton density. In this study, 51 species from 6 phytoplankton groups and 17 species and 4 larvae from 2 zooplankton groups were identified. The average abundance of phytoplankton was 4,229 cells per liter, while that of zooplankton was 10,395 cells per liter. The Bacillariaceae and Paracalanidae families, as well as the larval stages (Nauplius and Copepoda), were the dominant groups in the region. According to the evaluation of the WQS index, a rank of 2.6 was determined for the estuary stations and salt pond No. 1, while a rank of 3.4 was determined for the stations inside pond No. 2. According to the WQS index, the values of pH and dissolved oxygen during the sampling period received a score of 5 (good quality), while total nitrogen, total phosphorus, and BOD5 during the study period received a score of 1 (low quality). Overall, the stations at Odleh Creek and the beginning of pond No. 1 have low quality, whereas the stations at the end of pond No. 1 and inside pond No. 2 have poor quality in terms of water quality. A positive correlation was observed between phytoplankton abundance and salinity and nitrate levels, while zooplankton showed an inverse relationship with these two parameters.Discussion: According to the study, as salinity and nitrate increased in the ponds, the abundance of phytoplankton increased, while zooplankton decreased. This was probably due to the sensitivity of zooplankton to salinity and the creation of osmotic pressure beyond their capacity. Following this, with the reduction of predators and the availability of nitrate as a food source for phytoplankton, we witnessed greater growth of phytoplankton. Also, as we moved towards the ponds, species diversity decreased due to intense evaporation and high salinity, and resistant species formed the dominant population. On the other hand, according to the results of the WQS index, the water quality of the area was low, which can be attributed to stressful conditions such as intense evaporation. Therefore, in the ponds, it is necessary to conduct continuous monitoring and identify and eliminate stressful factors to improve the living conditions of aquatic animals in this area.