Seasonal distribution of water masses and their impacts on nutrient supply in the southern Beibu Gulf

Dual water isotopes were investigated to reveal the seasonal distribution of water masses and their impacts on nutrient supply in southern Beibu Gulf. In summer and winter, the South China Sea (SCS) water (61-69%) contributed the most to the seawater in the southern Beibu Gulf, followed by the diluted water (24-34%), and the west-Guangdong coastal current (WGCC) (5-7%) had the minimum contribution. However, the major nutrient source shifted from the diluted water in summer (39-73%) to the SCS water (57-90%) in winter. The WGCC's impact on nutrient loads was relatively small (2-10% in summer, 4-34% in winter). Our results highlight the control of nutrient supply was the SCS water (winter) and diluted water (summer), with limited influence from the WGCC, providing new insights into the impact of water mass transportation and its nutrient supply in the Beibu Gulf.

δ13C and δ15N zooplankton isoscapes as trace of water masses and mesoscale activity in the Pacific Tropical-Subtropical Convergence off Mexico during June 2010

This research analyzes for the first time in the Pacific Tropical-Subtropical Convergence off Mexico during June 2010 the relationships among the δ13C and δ15N signal distribution in the bulk zooplankton, water masses, and mesoscale structures. The environmental analysis showed that the California Current Water (CCW) and Transitional Water (TrW) converge where the 22 °C isotherm and 34.6 g kg-1 isohaline were observed (22°N). Two cyclonic eddies were detected: one in CCW at 24°N about 70 km in diameter, the other one located in TrW at 21°N with 150 km in diameter. A cluster analysis defined three zooplankton isoscapes with significant differences. Isoscape 1, immerses in CCW - the lowest δ13C (-22.37 ± 0.89 ‰) and δ15N (9.89 ± 1.32 ‰) - showed temperature (19.86 ± 1.97 °C), salinity (34.08 ± 0.37 g kg-1) values, the deepest thermocline (77.93 ± 25.51 m) and oxygen minimum zone (164.78 ± 44.96 m). The CCW was relatively oligotrophic without important mesoscale features in the sampled time within the sampled area. Isoscape 2, a cyclonic eddy in CCW - relatively enriched zooplankton δ13C values (-16.78 ± 1.95 ‰) due to the entry of enriched coastal biota- reflected in high chlorophyll a and subsequently assimilated into zooplankton tissues. Isoscape 3, a cyclonic eddy immerses in TrW - the highest δ15N (13.14 ± 1.60 ‰), salinity (34.76 ± 0.19 g kg-1) and temperature (19.90 ± 2.27 °C) values and shallowest thermocline (32.27 ± 18.63 m), and oxygen minimum zone (66.82 ± 37.68 m) in depth. Likely, this 15N-enriched recycled nitrate was pumped from the deoxygenated subsurface to the surface, reflected in the zooplankton bulks. The results showed that the isotopic signals in bulk zooplankton may be a good water mass and mesoscale activity tracer.

A comparative study on source of water masses and nutrient supply in Zhanjiang Bay during the normal summer, rainstorm, and typhoon periods: Insights from dual water isotopes

Typhoons and rainstorms (rainfall >250 mm day-1) are extreme weather events that seriously impact coastal oceanography and biogeochemical cycles. However, changes in the mixing of water masses and nutrient supply induced by typhoons and rainstorms can hardly be identified and quantified by traditional methods owing to the complex hydrological conditions in coastal waters. In this study, we analysed a comparative data set of dual water isotopes (δD and δ18O), hydrological parameters, nutrients, and chlorophyll-a from three periods (normal summer, rainstorm, and typhoon periods) in Zhanjiang Bay, a typical semi-enclosed mariculture bay in South China, to address this issue. The results revealed a significant increase in contributions from freshwater during rainstorms and typhoons. Correspondingly, nutrient supplies from freshwater during these periods remarkably increased compared to the normal summer, indicating that heavy rainfall can transport substantial amounts of terrestrial nutrients into the bay. Furthermore, disparities in hydrodynamic processes between typhoon and rainstorm periods were notable due to inconsistencies in freshwater diffusion paths. During rainstorms, freshwater primarily diffuses towards the outer bay in the upper layer due to strong stratification and cannot form an ocean front. However, under intense external forces caused by the typhoon, high-salinity water intruded into the bay, and enhancement of vertical mixing disrupted stratification. The massive influx of freshwater column during the typhoon mixed with higher salinity seawater column in the bay led to the formation of an ocean front, which could retain contaminants. This study suggests that although both rainstorms and typhoons can discharge large quantities of terrestrial nutrients into Zhanjiang Bay, the front formed during the typhoon period impedes the contaminant transportation to open sea thereby deteriorating water quality and affecting mariculture activities within the bay.

Mesozooplankton communities related to water masses in the Persian Gulf and the Gulf of Oman

The spatial and temporal variations of mesozooplankton in relation to environmental parameters were investigated in the Persian Gulf and the Gulf of Oman. For this purpose, the physicochemical parameters were measured using a CTD probe and mesozooplankton were sampled vertically using a closing net aboard the RV Persian Gulf Explorer during two expeditions. Results showed a relationship between mesozooplankton assemblages and water mass distributions. Four mesozooplankton cluster station groups divided the sampled stations into masses, which was also confirmed by the PERMANOVA test. Overall, ninety taxa were identified that were dominated by copepods and characterized by small cyclopoids. Based on the results the mesozooplankton communities of the Persian Gulf and the Gulf of Oman are mostly shaped by the combined effect of temperature, salinity, and dissolved oxygen that significantly influences mesozooplankton occurrence during summer and spring.

Benthic biogeographic patterns on the deep Brazilian margin

The Brazilian continental margin (BCM) extends from the Tropical to the Subtropical Atlantic Ocean, with much of its seafloor within deep waters, supporting rich geomorphological features and under wide productivity gradients. Deep-sea biogeographic boundaries on the BCM have been limited to studies that used water mass and salinity properties of deep-water masses, partly as a result of historical under sampling and a lack of consolidation of available biological and ecological datasets. The aim of this study was to consolidate benthic assemblage datasets and test current oceanographic biogeographical deep-sea boundaries (200-5,000 m) using available faunal distributions. We retrieved over 4,000 benthic data records from open-access databases and used cluster analysis to examine assemblage distributions against the deep-sea biogeographical classification scheme from Watling et al. (2013). Starting from the assumption that vertical and horizontal distribution patterns can vary regionally, we test other schemes incorporating latitudinal and water masses stratification within the Brazilian margin. As expected, the classification scheme based on benthic biodiversity is in overall agreement with the general boundaries proposed by Watling et al. (2013). However, our analysis allowed much refinement in the former boundaries, and here we propose the use of two biogeographic realms, two provinces and seven bathyal ecoregions (200-3,500 m), and three abyssal provinces (>3,500 m) along the BCM. The main driver for these units seems to be latitudinal gradients as well as water mass characteristics such as temperature. Our study provides a significant improvement of benthic biogeographic ranges along the Brazilian continental margin allowing a more detailed recognition of its biodiversity and ecological value, and also supports the needed spatial management for industrial activities occurring in its deep waters.

Biogeography, diversity and environmental relationships of shelf and deep-sea benthic Amphipoda around Iceland

The waters around Iceland, bounding the Northern North Atlantic and the Nordic seas, are a region characterized by complex hydrography and seabed topography. This and the presence of the Greenland-Iceland-Faroe-Scotland ridge (GIFR) are likely to have a major impact on the diversity and distribution of the benthic fauna there. Biodiversity in this region is also under increasing threat from climate-induced changes, ocean warming and acidification in particular, affecting the marine realm. The aim of the present study was to investigate the biodiversity and distributional patterns of amphipod crustaceans in Icelandic waters and how it relates to environmental variables and depth. A comprehensive data set from the literature and recent expeditions was compiled constituting distributional records for 355 amphipod species across a major depth gradient (18–3,700 m). Using a 1° hexagonal grid to map amphipod distributions and a set of environmental factors (depth, pH, phytobiomass, velocity, dissolved oxygen, dissolved iron, salinity and temperature) we could identify four distinct amphipod assemblages: A Deep-North, Deep-South, and a Coastal cluster as well as one restricted to the GIFR. In addition to depth, salinity and temperature were the main parameters that determined the distribution of amphipods. Diversity differed greatly between the depth clusters and was significantly higher in coastal and GIFR assemblages compared to the deep-sea clusters north and south of the GIFR. A variety of factors and processes are likely to be responsible for the perceived biodiversity patterns, which, however, appear to vary according to region and depth. Low diversity of amphipod communities in the Nordic basins can be interpreted as a reflection of the prevailing harsh environmental conditions in combination with a barrier effect of the GIFR. By contrast, low diversity of the deep North Atlantic assemblages might be linked to the variable nature of the oceanographic environment in the region over multiple spatio-temporal scales. Overall, our study highlights the importance of amphipods as a constituent part of Icelandic benthos. The strong responses of amphipod communities to certain water mass variables raise the question of whether and how their distribution will change due to climate alteration, which should be a focus of future studies.

Spatial distribution of the microzooplankton communities in the northern South China Sea: Insights into their function in microbial food webs

The spatial distribution of microzooplankton in the northern South China Sea was investigated in March 2016. Microzooplankton communities were dominated by cyclotrichids, aloricate oligotrichs, and choreotrichs within ciliates and the order Gymnodiniales within dinoflagellates. Microzooplankton abundance varied between 60 and 166,520 cells L^-1, with higher values in the coastal diluted water, and microzooplankton biomass exhibiting a similar pattern. High densities of Akashiwo cf. sanguinea were found in the upper waters along the coast, and mixotrophs dominated the communities in all the water masses. A canonical analysis of principal coordinates showed that the spatial patterns of microzooplankton communities could be clearly discriminated in the different water masses. Our findings provide insights into the functioning of microzooplankton and the potential risk of harmful Akashiwo cf. sanguinea algal blooms in coastal waters. In addition, our study provides evidence for using microzooplankton communities as potential indicators of water masses in complex marine systems.

Spatial distribution patterns of planktonic ciliate communities in the East China Sea: Potential indicators of water masses

The spatial distribution of planktonic ciliates over the coastal and continental shelf of the East China Sea were investigated using quantative protargol staining. Aloricate oligotrichs and choreotrichs were dominant in terms of species number, abundance and biomass. Ciliate densities varied between 3 and 2688 cells L^-1 with higher values occurring in the coastal water and the mixing water than in the Yellow Sea coastal water and the Taiwan warm water. Ciliate biomass exhibited a similar pattern as abundance. A canonical analysis of principal coordinates demonstrated that the spatial patterns of ciliate community structure could be clearly discriminated in different water masses. Diversity parameters showed strong relationships with spatial changes in ciliate communities and might serve as predictors of water mass in future studies. Our findings provide evidence for using ciliate communtiy composition, supplemented with dominant species and diversity parameters, as potential indicators of water masses in complex marine environments.

Sources and coastal distribution of dissolved organic matter in the Gulf of Cadiz

Dissolved organic matter (DOM) is a major component of the organic matter pool, playing a key role in the global ocean functioning. However, studies on DOM in waters of many ocean regions, such as the Gulf of Cadiz (GoC), are poorly known. Advanced aquatic sensors enable autonomous for long-term deployments in situ collection of high frequency DOM data using fluorescent dissolved organic matter (FDOM) as a proxy. The present study evaluates the relevance of FDOM, the estuarine influence and the environmental factors that determine its spatial distribution in the GoC. Our results suggest that the GoC water mass, under the estuarine influence of three main rivers, is receiving large amounts of DOM transported mainly by Guadalquivir and Guadiana rivers and much less from Tinto-Odiel. Salinity is the main factor explaining the FDOM variability within the Guadalquivir and Guadiana rivers and in the inner shelf of the GoC. In the outer shelf of the GoC, plankton-produced DOM could explain the persistent spatial pattern of FDOM, playing an important role in the dynamics of FDOM from the North area of the GoC through the persistent low-salinity Eastern North Atlantic Central Water. The oceanographic dynamics and the spatial pattern of FDOM concentration in the continental shelf of the GoC suggest a net transport of FDOM through the GCC (Gulf of Cadiz Current) to the Mediterranean Sea.

Anthropogenic organochlorine compounds as potential tracers for regional water masses: A case study of estuarine plume, coastal eddy, wind-driven upwelling and long-range warm current

Water masses are the crucial factor driving the terrigenous anthropogenic organochlorine compounds (OCs) migration from the coast to open sea. Therefore, organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) were investigated in the Northern South China Sea (NSCS), where different types of water masses are generated by the East Asian summer monsoon: Pearl River estuary plume (PREP), Guangdong offshore eddy (GDEC), South China Sea warm current (SCSWC) and wind-driven upwelling current (WDUC). No discrepant distributions of OC concentrations were found in these water masses (p > 0.05). However, compositions and diagnostic ratios of HCHs, DDTs, trans- or cis-chlordane and PCBs could reflect the discrepancies in the input, transport and transformation of OCs caused by the hydrological characteristics of water masses, therefore, this allowing them to serve as potential tracers of regional water masses. In detail, α/γ-HCH and β-HCH percentages could indicate the weathered residue in the GDEC, long-range transport in the SCSWC, rapid photodegradation in the surface WDUC and biodegradation in the deep WDUC, respectively. The predominance of o, p'-DDT and p, p'-DDT could indicate fresh input in the PREP, GDEC and WDUC. DDT/DDTs of ratios <0.5 also reflected long-range transport in the SCSWC. Different DDD/DDE ratios indicated different oxygen environments of microbial degradation in the surface and deep water of the WDUC. Trans/cis-chlordane ratios could indicate the selective degradation of trans-chlordane in different water masses. Finally, a higher proportion of penta-PCB could reflect the strong paint additive sources carried by river erosion in the PREP.