Saccular otolith mass asymmetry is examined in three sparid fish species, Acanthopagrus bifasciatus, A. arabicus, and Sparidentex hasta collected from Khor Abdullah at the North Persian Gulf. This characteristic was computed as the disparity between the weight of the right and left otoliths divided by mean otolith weight in the three sparid species investigated. According to the previous cases obtained on another symmetrical fish species, the absolute value of x in these species does not determine by fish length and otolith growth ratio, while the absolute rate of otolith weight disparity is boosted with the fish length. The estimate of x was between -0.2 and +0.2. Otolith mass asymmetry can show some growth disorder of fish owing to environmental influence.

Due to the ecotoxicological effects, microplastics are considered a threat for the marine environment. Recent reports indicate their presence not only in subsurface water and in coastal beach sediments, but also in the deep-sea. Notwithstanding, there is still not a scientific consensus about the analytical procedure to be applied for their determination. In this work we compared the performance of two extraction methods: pressurized solvent extraction (PSE) and density separation. Sea sand and seafloor sediments were spiked with known amounts of polystyrene (PS), polyethylene (PE) and polypropylene (PP) microplastics and submitted to both the extraction procedures. Results showed that the PSE ensured higher recoveries for the smaller size particle fractions (89,2 ± 1.1% in the 50–150 μm range) whereas the density separation enabled precise recoveries for the larger size particles (SD = 1,5%). No significant differences in terms of blanks control were highlighted.

Microplastic pollution is a growing, yet poorly understood problem. Here, we assessed the relationship between microplastic concentration and distance to rivers, shorelines, cities, sediment grain size or water depth in sediments of the world's largest (semi-)enclosed aquatic basins. Microplastic was extracted from sediment using density separation, elutriation and hydrophobic adhesion. Fibers and transparent or white microplastic particles were the most abundant shape and color. The microplastic concentration in sediments of the Black Sea was about twice as high compared to that in the Caspian Sea. Fragment concentrations decreased with depth, while fiber concentrations were independent of depth. Overall, no relationship with distance to shores, rivers and cities or with grain size was observed. However, within some depth classes concentrations were related to the distance from rivers, shores and cities.

The ubiquity of microplastics raises issues regarding contamination control measures and laboratory practices. The objective was to adapt the use of counting chambers and plastic microplates on the ecotoxicity evaluation of microplastics. Counting chambers, originally used to quantify cells, can also be used to count high concentrations of microplastics (<100 μm) used in laboratory assays. By decontaminating the chamber and mixing the test solution with Nile Red (1:1), fluorescent particles can be easily counted under optical microscopy. Microplate wells, due to their composition, can be contaminated or release microplastics to the test medium, which can interfere with the results of ecotoxicity assays or spectroscopy readings. A cleaning method based on ethanol was developed, which effectively removed particles by 91% without interfering with microalgae yield. Besides providing practical applications that can improve ecotoxicity assays, this work intends to raise awareness on the need to adapt laboratory practices when working with microplastics.

Microplastic pollution in marine environments has become a major global environmental issue. However, the source of microplastics in marine environments is poorly understood. Here, we investigated the distribution and the source of microplastics in surface seawater from Sanggou Bay, China. The results showed that the average abundance of microplastics was 20.06 ± 4.73 items/L, which represented a medium and high level relative to other sea areas. Microplastics of <0.5 mm dominated. Most particles were granular and transparent, and polyethylene was the dominant polymer. We estimated that 62.76% of the microplastics originated from mariculture facilities, an indication that mariculture contributes significantly to microplastics pollution, and calculated that 96.15 kg was present in surface seawater of Sanggou Bay. Our findings indicated that more attention should be paid to mariculture-derived microplastics pollution in the future.

Mercury (Hg) concentrations based on dietary sources have shown to predict differences in fish; however, they are usually applied at an individual scale and are rarely directed at a known trophic transfer. We combined gut content analysis and stable isotope analysis (δ¹⁵N and δ¹³C) to provide a quantitative estimate of Hg and selenium (Se) biomagnification in the striped marlin (Kajikia audax) and blue marlin (Makaira nigricans) food web from the southwestern Gulf of California. Hg and Se concentrations (mean ± SD; μg g⁻¹, dw) were different among K. audax (Hg = 3.6 ± 2.1, Se = 5.5 ± 5.4) and M. nigricans (Hg = 19.0 ± 29.6, Se = 8.8 ± 10.5). Such variations of element concentrations could be linked to predation with different Hg and Se contents. Diet data presented as prey weight (%W) indicated a higher proportion of large prey fish for the blue marlin than the striped marlin. δ¹⁵N and δ¹³C indicated pelagic food sources with epipelagic preferences for the blue marlin and mesopelagic for the striped marlin. The relationship between Hg concentrations and δ¹⁵N was positive along the food web of both marlin species, indicating biomagnification of Hg. However, Se biomagnification was not clearly evidenced, and Se:Hg ratios decreased with δ¹⁵N, attributed to increasing Hg concentrations with increased trophic level.

The Beibu Gulf is an important passageway between China and the Association of Southeast Asian Nations, where there has been an increase in pollution of heavy metals (HMs). High concentrations of Pb, Cr, Cd, Cu, Zn, As, and Hg in surface sediment were found in Qinzhou Bay, Fangchenggang, and other coastal areas. Stochastic geo-accumulation analyses identified the pollution to be “uncontaminated”; however, it had an 18% probability of deterioration. The Cd, Hg, and As pollution were relatively serious. Principal component analysis, positive matrix factor model, and mercury isotopes demonstrated that the HMs could mainly be attributed to industrial sources including petrochemical, coal-fired, metal and metalloid processing, leather tanning, and human activities: anthropogenic sources accounted for approximately 70% of all the contaminations. This study demonstrates the contribution of terrigenous input to HMs even at a low level and provides basic data for the coordinated development of land and marine resources.

Mercury (Hg) concentrations have significantly increased in oceans during the last century. This element accumulates in marine fauna and can reach toxic levels. Seafood consumption is the main pathway of methylmercury (MeHg) toxicity in humans. Here, we analyzed total Hg (T-Hg) concentrations in two oceanic squid species (Ommastrephes bartramii and Thysanoteuthis rhombus) of an increasing commercial interest off Martinique, French West Indies. Stable isotope ratios reveal a negative linear relationship between δ¹⁵N or δ¹³C in diamondback squid samples. No significant trend was observed between δ³⁴S values and T-Hg concentrations, contrasting with the sulfate availability and sulfide abundance hypotheses. This adds to a growing body of evidence suggesting Hg methylation via sulfate-reducing bacteria is not the main mechanism driving Hg bioavailability in mesopelagic organisms. All squid samples present T-Hg levels below the maximum safe consumption limit (0.5 ppm), deeming the establishment of a commercial squid fishery in the region safe for human consumption.

Bioindicators are useful for determining nutrient regimes in marine environments, but their ability to evaluate corals reefs in different ecological states is poorly understood. The precision, availability and congruency of eight potential bioindicators (brown macroalgae, green macroalgae, turf algae, cyanobacteria, soft corals, zoanthids, sponges, and sediment) and their stable isotopic and elemental signatures (δ¹⁵N, δ¹³C, %N, %C, and C:N Ratio) were assessed across 21 reefs in the Inner Seychelles. The coefficient of variation (CoV) for δ¹⁵N showed that green and brown macroalgae were highly precise (2.47 ± 0.95, n = 11; 4.68 ± 1.33, n = 16, respectively), though were less common on coral-mortality reefs relative to macroalgal-dominated ones. Zoanthids were also highly precise for δ¹⁵N (2.98 ± 1.20), but were more readily available regardless of reef state (n = 18). Congruency was low among these indicators, suggesting that different physiological mechanisms for nutrient processing have a stronger influence on a bioindicator's effectiveness than reef state.

To evaluate the trace metals contamination status in the Gulf of Tunis, forty one sediment samples were analyzed using different approaches. According to certain contamination and ecological risk indices (Contamination Factor, Geoaccumulation index and Ecological risk index), Hg has the highest contamination level while pollution by Ni, Pb, Cd and Cr was absent. The highest concentrations of trace metals were found in sediments collected from the offshore and coastal areas located opposite the main exchange points with the gulf particularly, the Mejerda and Meliane Rivers, the Khalij Channel, Ghar El Melh and El Malah lagoons, Tunis Lake and Sebkhat Ariana. However, further ecological indices (Potential ecological risk index, Toxic unit and Mean effect-range median quotient) and comparison with sediment quality guidelines suggest that in addition to Mercury, Cr, Pb and Ni concentrations are detrimental to biota in both the offshore and areas near to the exchange points with the gulf. Moreover, in these areas the results from sequential extraction and individual contamination factor calculation pointed to the mobility and bioavailability of Cr, Pb and Ni.