Evaluation of hydrochemical parameters in coastal waters of Chabahar: seasonal variations and diurnal fluctuations

Environmental parameters and their short and long-term variations are widely studied to monitor the changes in coastal water quality. Dynamic changes in parameters such as temperature, pH, DO, and nutrients are closely related to primary production by autotrophic species and their population structure. The availability of nutrients and their ratio is considered the key factors regulating the biomass of phytoplankton (Roelke, Eldridge, & Cifuentes, 1999). The growth and metabolism of these organisms are strongly influenced by nitrogen, phosphorus, and silicon as nutrients. The biological response to environmental changes is closely related to the periodicity of fluctuations in these parameters. While the variation in seasonal scale is influenced by the seasonal pattern of currents and meteorological parameters, on a diurnal scale, they are influenced by the processes of photosynthesis and decomposition of organic matter. These fluctuations in hydrochemical parameters can shape the population structure of phytoplankton, the outbreaks of harmful species, and the expansion of low-oxygen regions in the upper layers of the water column in the post-bloom phase. Therefore, the trophic status is evaluated in various ways, including the concentration of nutrients or chlorophyll-a (Carlson & Simpson, 1996).In the northern regions of the Gulf of Oman, meteorological conditions and water column characteristics are mostly governed by the summer (June-August) and winter (December-March) monsoons. Most of the summer monsoon's impact on hydrochemistry and function of biological life is through local wind-driven or eddy-induced upwelling in coastal areas, during which, colder nutrient-rich waters rise to the surface from depths (Harrison, Piontkovski, & Al-Hashmi, 2017). During the winter monsoon, cool convective mixing increases the mixing layer depth and helped nutrients to reach the surface (Harrison et al., 2017).The frequent occurrence of harmful algal blooms and the establishment of hypoxic conditions as a result of the increase in the concentration of nutrients have affected the coastal waters of Chabahar in recent years (Ershadifar et al., 2020; Kor, Ghazilou, Ershadifar, & Koochaknejad, 2020). On the other hand, Gulf of Oman, as a part of the northwest of the Arabian Sea, has the potential to be considered globally as a sensitive indicator of global climate change at the oceanic scale (Fauzi et al., 1993). The region is host to various biogeochemical phenomena such as the upwelling of nutrient-rich waters, nitrogen fixation, and nitrogen deficiency, as well as deoxygenation in both coastal and offshore areas.

In the present study, the short-term fluctuations (diurnal) as well as seasonal variations in hydrochemical parameters including salinity, temperature, pH, dissolved oxygen, and nutrients (nitrate, silicate, and phosphate) were investigated in Chabahar coastal waters. Sampling was done from a single station (depth of 30 m) on October 9 of 2018, November 19 of 2018, January 07 of 2019, and March 10 of 2019. The samples were collected in time intervals of 3 h and from three depth levels of 0, 10, and 30 m using a Niskin bottle each in three replicates. Environmental parameters including temperature, salinity, DO and pH were recorded by using portable devices at the sampling site. The samples were stored in a freezer at -20°C and analyzed for a short period of time. In the laboratory, nutrients concentration were measured according to the instructions for the analysis of nutrients in marine environments (Hansen & Koroleff, 1999). The double beam Rayleigh UV-920 spectrophotometer with 2 cm pathlength was used for colorimetric determination. Measurement of total phosphorus and nitrogen in unfiltered samples was also done by wet oxidation method in the presence of persulfate as an oxidizing agent at high-temperature conditions followed by the spectrophotometric determination of nitrate and phosphate (Grasshoff, Kremling, & Ehrhardt, 1999). To measure chlorophyll-a, the samples were extracted in 90% acetone and measured by the spectrophotometric method according to ESS Method 150.1 (EPA, 1991). The Ocean Data View (odv 5.1.7) software was used to depict the temporal-vertical fluctuations (Schlitzer, R., 2020, Ocean Data View, http://odv. awi.de). The General Linear Model (GLM) followed by Tukey's posthoc tests (Minitab V17 software) was used to evaluate the seasonal changes in the investigated parameters.

Continuous monitoring of coastal water quality is required for the sustainable development of cage culture projects, marine ecotourism, and desalination plants, and also to protect the sensitive ecosystems of the region, including coral reefs. The diurnal and vertical variations in the distribution of hydrochemical parameters in the coastal waters of Chabahar are affected by the changes in the water column structure varied from strong stratification in the fall intermonsoon monsoon to mixed column in the winter monsoon. As the temperature declined, the seasonal thermocline established in October completely faded in January, and the vertical gradients in hydrochemical parameters weakened or completely faded. Dissolved oxygen and pH, which have experienced low values in the inter-monsoon period, more profoundly in the near bottom layer, were affected by the processes of aerobic remineralization of organic matter. They were increased in the winter monsoon and there was no significant difference between the surface and near bottom values. Surface nutrients, with low concentrations during the stratification period, showed an increase toward winter monsoon. However, the average values of silicate, nitrate, and nitrogen over the water column showed a general decreasing trend from October to March. As a result of the increase in water column mixing, Chl-a also shows its maximum in March. The ratio of nitrate to phosphate is less than 10 in 90% of samples. The average value of the nitrate: phosphate ratio was 5.0, which was much lower than the theoretical value of 16 suggested by Redfield for offshore open ocean waters. This could imply a higher probability of nitrogen limitation rather than phosphorus limitation. In 45% of the samples, the silicate concentration was less than 2.0 µM, including the surface samples in October and November and almost all samples from March, which showed the active removal by diatoms. The diurnal fluctuations of the parameters, especially during the stratification period of the water column, are influenced by tidal and non-tidal currents so that the maximum and minimum values in a diurnal cycle could appear in both low and high-tide situations.