engleski

Picoplankton community dynamics in Špinut and Strožanac ports

It is well known that anthropogenic impacts have significant effects on marine communities. One of the consequences is the increased input of anthropogenic substances into the coastal zone which affects the fragile marine environment through various mechanisms, including high inorganic nutrient loads, organic compounds, sediments and the uptake of heavy metals. Recognition of port waters in coastal ecosystem monitoring programs is increasing as economically important infrastructure is embedded in the coastal environment and its management needs to be considered. General knowledge of the ongoing processes in these waters, which play a strategic role in the coastal environment needs to be acquired, considering abiotic and biotic factors, to implement sustainable development (Blue Growth) in port areas. Typically, the regulation implemented in ports and adjacent areas refers to chemical emissions and the consequent pollution of water and sediments together with sanitary aspects related to microbial pathogens, as basic controls to protect human health. Physicochemical and biotic parameters in the water column are subject to higher variability on shorter temporal scales compared to benthic parameters, related to seasonal changes and inflow/outflow from associated marine and inland waters. Planktonic taxa respond rapidly to environmental perturbations, although they have rarely been considered as descriptors of marine ecological quality. Particularly, bacterioplankton abundance and activity have been recognized as sensitive sentinels of environmental change. From an ecological perspective, bacterioplankton, phytoplankton and zooplankton are the most important planktonic components in the water column in terms of abundance, biomass, diversity, trophic networks and ecosystem services provided. Marine microbes are a diverse group of organisms including bacteria, archaea and eukaryotic protists that drive most biogeochemical cycles and play a central role in the transformation and recycling of carbon and energy in the marine environment. Bacteria dominate the abundance, diversity and metabolic activity of the ocean. Autotrophic picoplankton forms the base of the trophic food chain, while heterotrophic bacteria (HB) use almost exclusively the dissolved fraction of primary production and act as decomposers and transformers of compounds into forms suitable for primary producers. Heterotrophic nanoflagellates (HNF) participate in the material and energy flow towards higher trophic levels in the ecosystem through bacterial predation. Studies on inhabiting biota in ports are scarce. The present study aimed to determine the dynamics of the picoplankton community to gain insights into the ecology of two Croatian ports, Špinut and Strožanac. We hypothesized that there would be a difference in the bacterioplankton counts between the investigated stations and the control points in the two ports. Maritime Sports Society Špinut, located in the north of Split, has a total of 950 berths, 350 of which are for boats less than 6 m long. Many freshwater springs influence the water temperature and salinity in winter and spring. Sport Fishing Society Strožanac, with 330 berths for various boat sizes, is located near the beach in Podstrana and the mouth of the river Žrnovnica. Submarine freshwater springs also influence the water temperature and salinity in this area. In this study, sampling campaigns were conducted in the surface layer (30 cm below the surface) monthly for a period from April 2019 to January 2021. Flow cytometric counts of heterotrophic and autotrophic picoplankton, including the cyanobacteria Synechococcus (Syn) and Prochlorococcus (Pro) and picoeukaryotes (PE), were conducted. The Gasol model was used to determine grazing on HB by HNF, i.e., the flux of bacterial biomass to higher trophic levels. To test our hypothesis, we used the paired-samples t-test on HB. In general, abundances of picoplankton were more evenly distributed in Špinut than in Strožanac port throughout the investigated period. Maximum values in both ports were higher in Strožanac, except for HB whose values were similar (1.7 ᵡ 106 cell mL-1) and HNF, whose values were an order of magnitude higher in Špinut (23.85 ᵡ 103 cell mL-1 in April 2020). However, the median values were higher in Špinut than in Strožanac, except for Syn. This indicates that both investigated ports are not under anthropogenic pressure as the values of HB abundances rarely exceed 106 cell mL-1 (median 0.3 and 0.4 ᵡ 106 cell mL-1), which is a limit for eutrophication. The Gasol model showed that HNF grazed mainly on HB during the studied period with some exceptions. Specifically, HNF was “top-down” predated in Špinut partly in winter 2020 and autumn and in Strožanac partly in winter 2020 and summer, while HB was controlled by some other factors at that time. Carbon stored in bacterial biomass is thus transferred to higher trophic levels, which could further enhance the early developmental stages of pelagic fish. A paired t-test revealed a statistically significant difference between the studied and control points in Špinut port (P≤0.05) as predicted. Meanwhile, there was no statistically significant difference between two out of six studied points and the control point in Strožanac (P<0.05 for four stations and P=0.12 and P=0.56 for the other two stations). The observed differences are expected as Špinut is a rather enclosed port in Kaštela Bay, while Strožanac is a small port that communicates more with the outer sea, especially the two sampling points that displayed no difference with the control point. Taken together, the above results provide one of the few insights into microbial communities in port ecosystems in general. We confirmed the hypothesis that a small statistically significant difference is found in Špinut and Strožanac ports compared to control points. Therefore, these ports do not contribute to anthropogenic pressures on the adjacent coastal environment.

picoplankton dynamic, heterotrophic nanoflagellate, grazing, anthropogenic pressure

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