Mesozooplankton assemblages and their relationship with environmental variables: a study case in a disturbed bay (Beagle Channel, Argentina)

Diel variation of plankton in the highly impacted freshwater zone of Hooghly estuary in relation to ecological alteration

Plankton are promising ecological monitoring tool that responds quickly to any sort of aquatic ecological alteration, of which many of them are much susceptible to ecological variations. Therefore, monitoring shifts in plankton composition can indicate changes in water quality and aid to identify potential pollution sources. In the present study, the variation in plankton dynamics in relation to ecological variables were monitored in the freshwater zone of the Hooghly estuary from May 2020 to April 2021. The study was conducted in the interval of every six hours. i.e., at 6 A.M., 12 P.M., 6 P.M., and 12 A.M. The present finding revealed the occurrence of 54 phytoplankton and 20 zooplankton taxa/species. Diel variation revealed that among different time intervals, the highest abundance of phytoplankton was recorded 28,307 cells l-1 at 12 P.M, while the lowest was recorded 10,632 cells l-1 at 6 A.M. However, the highest zooplankton abundance was observed 804 ind l-1 at 6 A.M., and the lowest was recorded 156 ind l-1 at 6 P.M. The ANOVA (p < 0.05) analysis indicated significant diel variation for many planktonic genera. The CCA exhibited that most of the phytoplankton were influenced by multiple water quality variables such as temperature, turbidity, calcium, pH, salinity, DO, and nutrients. However, the majority of the zooplankton were affected by turbidity, total phosphorus, sulphate, calcium and available nitrogen. Significant seasonal variation in plankton composition has also been observed. The present study will help to determine the varying diel pattern of planktons in retort to alterations in the water quality parameters and varying ecological niches.

Water Circulation and Transport Time Scales in the Beagle Channel, Southernmost Tip of South America

The Beagle Channel is a long and narrow interoceanic passage within the Tierra del Fuego archipelago in the southernmost tip of South America. A high-resolution 3D hydrodynamic model based on the finite elements method was applied to investigate the residual circulation, water fluxes and transport time scales inside this channel. Numerical solutions were analyzed at seasonal time scale and the model results compared with observed ocean data. The circulation pattern is characterized by a west-to-east residual flow with low intensity and low seasonal variability. The water fluxes through the channel were estimated to be, on average, around 12,700 m3/s, with inflow through its western entrance and eastwards outflow mainly through the Mackinlay Strait. The water residence times vary seasonally with basin averages between 36 and 43 days and maximum values between 53 and 95 days. The results provide an overview of the hydrodynamics and water residence times in the Beagle Channel, a unique ecosystem threatened by recent anthropogenic pressures and climate change.

Variability in the zooplankton assemblages in relation to environmental variables in the tidal creeks of Sundarbans estuarine system, India

The present study illustrates a holistic account of zooplankton community dynamics in relation to physicochemical variables in the tidal creeks of Indian Sundarbans estuarine system. Eleven water variables and zooplankton samples were collected in a monthly basis from the six sampling locations between July 2016 and June 2018. Out of the 11 water parameters, seven parameters (temperature, salinity, dissolved oxygen, turbidity, PO₄ - P, NO₃ - N, and NO₂ - N) varied significantly (p ≤ 0.05) among seasons. A total of 63 zooplankton taxa were recorded with the predominance of Copepoda. Permutational multivariate analysis of variance (PERMANOVA) depicted the significant variations of zooplankton population both spatially (F = 2.313; p = 0.001) and temporally (F = 6.107; p = 0.001). Out of the 41 species of Copepoda recorded, 14 species (Paracalanus parvus, Parvocalanus dubia, Bestiolina similis, Acrocalanus gibber, A. gracilis, Acartia erythraea, A. spinicauda, Pseudodiaptomus serricaudatus, P. annandalei, P. aurivilli, Oithona brevicornis, O. similis, Longipedia weberi and Microsetella norvegica) were indicated as "characterizing species" in the creek environment, which highlighted the euryhaline nature as well as broad range of thermal tolerance of these species. Acrocalanus longicornis, Eucalanus crassus, Candacia bradyi, and Acartia sewelli were specifically found in pre-monsoon; Euterpina acutiformis, Clytemnestrata rostrata, Acartia plumosa, and Canthocalanus pauper were specific to post-monsoon season, whereas Mesocyclops leuckarti, Temora turbinata, and Temora discaudata were found only during monsoon. Thus, it explains the limited period of existence, highlighting the temporal shift in species abundance in the creeks. Overall, the calculated mean value of α-diversity (d' = 4.07; H' = 2.31) indicated a "good" zooplankton diversity in the creeks. The biota-environmental relationship following canonical correspondence analysis revealed that variables, viz., water temperature, turbidity, and NO₃ - N, were the explanatory variables during monsoon; pH and DO in post-monsoon; PO₄ - P, SiO₄ - Si, and salinity in pre-monsoon that explained the variations of zooplankton community compositions in the creeks. The variables (PO₄ - P, NO₃ - N, and SiO₄ - Si) that are correlated to the zooplankton community implied their indirect effect on the distribution of zooplankton by supporting the phytoplankton succession and production in the mangrove-dominated creeks.

Influence of tidal forcings on microphytobenthic resuspension dynamics and sediment fluxes in a disturbed coastal environment

The resuspension dynamics of microphytobenthos (MPB) and sediment fluxes were investigated in a disturbed coastal environment by employing an in situ mooring system. We aimed to identify the interrelationship between microalgal biomass and sediment particles in aspect of their (de)coupling mechanism in a tidal channel system. In specific, the Chl-a (as a proxy of MPB biomass) and the suspended sediment concentration (SSC) were simultaneously measured under natural tidal conditions at different time scales, encompassing daily to fortnightly variations. Looking for the sediment dynamics, the SSC showed a strong positive correlation (p < 0.001) with the Reynolds stress; however, this relationship was not observed for benthic Chl-a. This could be due to more dynamic characteristics of the smaller biological cells, i.e., the decoupled benthic Chl-a from the sediment particles might randomly distribute in the turbid water column above the tidal channel. Notably, the iteration between MPB coupling (prevailing in spring) and segregation (prevailing in neap) with the sediment particles across the flood and ebb tidal cycles was evident during the study period. Meantime, the onshore flux of suspended sediment was almost balanced by its offshore flux, but that of Chl-a appeared to be unbalanced due to an excessive onshore transport. Altogether, the study area seems to experience a time lag in resuspension between MPB and sediment, followed by biological trapping in the tidal channel system, which would support a productive shallow water environment. The present study is the first to address the tidal resuspension of benthic microalgae in relation to sediment dynamics in a disturbed coastal environment of the Yellow Sea.

The Copepod Acartia tonsa Dana in a Microtidal Mediterranean Lagoon: History of a Successful Invasion

The Lagoon of Venice has been recognized as a hot spot for the introduction of nonindigenous species. Several anthropogenic factors as well as environmental stressors concurred to make this ecosystem ideal for invasion. Given the zooplankton ecological relevance related to the role in the marine trophic network, changes in the community have implications for environmental management and ecosystem services. This work aims to depict the relevant steps of the history of invasion of the copepod Acartia tonsa in the Venice lagoon, providing a recent picture of its distribution, mainly compared to congeneric residents. In this work, four datasets of mesozooplankton were examined. The four datasets covered a period from 1975 to 2017 and were used to investigate temporal trends as well as the changes in coexistence patterns among the Acartia species before and after A. tonsa settlement. Spatial distribution of A. tonsa was found to be significantly associated with temperature, phytoplankton, particulate organic carbon (POC), chlorophyll a, and counter gradient of salinity, confirming that A. tonsa is an opportunistic tolerant species. As for previously dominant species, Paracartia latisetosa almost disappeared, and Acartia margalefi was not completely excluded. In 2014–2017, A. tonsa was found to be the dominant Acartia species in the lagoon.

Biogeochemical markers across a pollution gradient in a Patagonian estuary: A multidimensional approach of fatty acids and stable isotopes

A combined approach merging stable isotopes and fatty acids was applied to study anthropogenic pollution in the Río Negro estuary. Fatty acid markers of vegetal detritus indicated considerable allochthonous inputs at freshwater sites. Correlative evidence of diatom fatty acids, δ¹³C, chlorophyll and particulate organic matter suggested the importance of diatoms for the autochthonous organic matter production at the river mouth. Low δ¹⁵N values (~0‰) and high fatty acid 18:1(n-7) concentrations in the suspended particulate matter, in combination with the peaks of coliforms and ammonium, indicated a strong impact of untreated sewage discharge. The ¹⁵N depletion was related to oxygen-limited ammonification processes and incorporation of ¹⁵N depleted ammonium to microorganisms. This work demonstrates that the combined use of lipid and isotopic markers can greatly increase our understanding of biogeochemical factors and pollutants influencing estuaries, and our findings highlight the urgent need for water management actions to reduce eutrophication.