The complexity of the marine environment and the increasing anthropogenic pressure create a necessity to expand existing monitoring approaches. The main goal of this study was to depict the effects of selected, seasonally varying environmental factors on a battery of biomarkers in the benthic amphipod Monoporeia affinis from the Gulf of Riga (GoR). Seasonal variability in acetylcholinesterase (AChE), catalase (CAT), glutathione reductase (GR), and glutathione S-transferase (GST) activities was investigated at six coastal stations (20–30 m) in August and November in 2020 and 2021. In addition, the biomarkers were measured at seven deep-water stations (>30 m) in November 2021. In general, the results indicated no significant influence of the measured environmental variables on the biomarker activities, except for deep-water stations, where chlorophyll a significantly affected enzymatic activity. The current study indicated that M. affinis has a higher GST, CAT and GR activity in summer compared to autumn in coastal stations, showing seasonal variability of these biomarkers. However, summarizing the biomarker levels recorded at each station and season, the integrated biomarker response (IBR) index showed the most stressed health status of the M. affinis populations in the deep-water stations 135 and 107 and coastal regions in the north-eastern part of the GoR (station 160B). This suggests that the impact on enzymatic responses of benthic organisms could be due to port activities leading to the accumulation of pollutants in muddy sediments regionally. Moreover, for the monitoring of biological effects of contaminants there is a need to establish the background levels of biomarkers, i.e., responses to the different natural environmental factors in the GoR region.

Chemical pollution affects the health of marine biota, key functions of the ecosystem, and endangers its biodiversity. The current environmental monitoring and assessment approach is based on chemical and ecological measurements, while the linkage between them, biological effects at the individual level, has been largely neglected. It is currently well acknowledged that the monitoring of chemical concentrations alone is not sufficient to protect populations; it considers only a tiny number of substances, while myriad others are left unnoticed. In addition, the hazards related to contaminant mixtures remain undetected. Biological effects of contaminants can be detected and measured at different levels of biological organization, i.e., from the molecular/biochemical level up to changes in populations and communities. In Baltic Sea monitoring programs, the few observations on the effects of contaminants have, for decades, mostly been made at only the higher biological organization levels, applying reproduction success parameters in top predators such as raptor birds and seals as indicators. However, recording early warning signals of contaminant exposure and effects on the health of individuals at lower biological levels makes it possible to anticipate and prevent damage at the higher levels. Moreover, it is also crucial to monitor effects at lower levels of the marine food web. The so-called biomarkers focus on changes in various biological functions and include parameters related to the detoxification of xenobiotics, oxidative stress, neurotoxicity, geno- and cytotoxicity and reproductive disorders, among others. Research on biomarkers aiming at improving their use in monitoring and assessing the contamination status of marine areas has been intensive during recent decades, although the implementation of the methods to monitoring programmes has been slow. Here, the current state of biological effects monitoring using biomarkers, as well as selected case studies in the Baltic Sea, are presented, followed by recommendations on their application in this sea area.

Over 38000 tons of chemical munitions were dumped in the Baltic Sea after World War II. They rest on the bottom of the Gotland and Bornholm Deeps. Studies performed between 2011 and 2019 show that those munitions are largely corroded, and chemical warfare agents (CWA) are released to the environment. Biomarker studies indicate, that they adversely affect marine biota from the dumpsites. Hydrodynamic models suggest that they can be transported with currents to adjacent areas. CWAs are transformed in the environment producing a variety of different degradation products, some of them characterized by higher toxicity than the parent compounds. An effort was undertaken to summarize the studies performed in three EU Interreg and one NATO SPS projects focusing on this problem.

Sulfate is a natural major ion commonly found in the environment. Various anthropogenic activities can lead to the discharge of sulfate in high concentrations, causing negative effects on aquatic organisms. While chronic toxicity of sulfate for brackish water species was established, acute toxicity data were scarce. In this study, we tested the acute toxicity of sulfate to seven Baltic Sea species and, together with the literature data, derived the fifth percentile hazardous concentration (HC5) through a species sensitivity distribution (SSD) with 12 species. Acute sulfate sensitivity varied across taxa, with fish and crustaceans being much more sensitive than bivalves, algae, rotifers and bacteria, which led to a bimodal SSD. The most sensitive species tested were whitefish (Coregonus lavaretus) and vendace (Coregonus albula) during fertilization, and the most tolerant were cyanobacterium Synechococcus sp. and bacterium Aliivibrio fischeri. Acute HC5 using the model-averaging approach was 2489 (1016–3990) mg/L sulfate. With a Predicted No Effect Concentration of 1011 mg/L, the upper limit of the 95% confidence interval for the estimated percentage of species affected was below 5%. This new data contributes to the development of the maximum allowable concentration for the sulfate environmental quality standard in the northern Baltic Sea.