In this study, we explored the impact of eutrophication and stratification on hypoxia in deep waters of moderately warm Croatian marine lakes. Although the Mljet Lakes (MLs) are predominantly ...oligotrophic, mesotrophic conditions are present at depths below 20 m in the Small Lake (SL) and below 30 m in the Big Lake (BL), along with higher apparent oxygen utilization (AOU). Hypoxia at depths ≥ 25 m in SL and and ≥ 40 m in BL was observed between October 2009 and January 2010, and in SL in summer (July and September 2010). Significant differences (p < 0.05) in several physical, biological and chemical parameters were detected between the lakes, while AOU, derived oxygen utilization rate (OUR) and organic carbon remineralization rate (OCRR) were not significantly different (p >0.05) between the lakes. An intense and persistent pycnocline throughout the year, comparatively high water temperature, extended water renewal time and summer phytoplankton bloom were identified as physical and biological parameters which might have significantly contributed to increased frequency of hypoxic events in a shallow SL. Significantly (p < 0.05) higher ammonium concentration in SL, especially in its deep water, seems to be a long-term chemical feature related to the poor ventilation and higher sediment oxygen demand. At the current level of eutrophication and the present climate change trends, the MLs and similar systems may experience more persistent and intense stratification, which could further prevent mixing between upper and deep waters, likely leading to increasing duration of hypoxia and its negative impacts on the biodiversity of benthic communities.
The aim of the present study was to investigate the diversity, distribution of virulence factor genes and antimicrobial resistance phenotypes of Vibrio Harveyi clade species from two distinct and ...spatially distant near-shore bivalve farming environments located in protected marine reserves of the eastern Adriatic. Tissues of farmed Mediterranean mussel Mytilus galloprovincialis and European flat oyster Ostrea edulis, seawater and sediment samples were collected during six bi-monthly sampling events conducted over one year from farming locations in Lim Bay and Mali Ston Bay, Croatia. Eight distinctive Harveyi subclades, namely, V. harveyi, V. alginolyticus, V. diabolicus, V. jasicida, V. parahaemolyticus, V. rotiferianus, V. campbellii and V. communis, were detected by Phylogenetic Multilocus Sequence Analysis (MLSA) based on rpoD, toxR and rctB concatenated genes sequences. Genomic fingerprinting of MLSA-classified samples revealed substantial diversity of environmental isolates. The virulence-related genes considered typical for the Harveyi clade and their homologues were represented in all subclades with varying frequencies. Of twelve antibiotics commonly used in human and veterinary medicine, the most frequent resistance phenotypes were resistance to ampicillin (90%), erythromycin (39%), imipenem (29%) and streptomycin (22%). >60% of isolates were resistant to at least two classes of antibiotics. Seasonal or site-specific differences were found in the proportion of isolates carrying virulence genes and displaying specific antibiotic resistance phenotype.
This study is the first to provide a broader perspective on the properties of Vibrio Harveyi clade species associated with ecosystems under presumably low anthropogenic pressures, such as marine protected areas. New data on virulence and antibiotic resistance are presented of Harveyi clade species coexisting in different environmental compartments, including those that have been largely overlooked or understudied in the context of bivalve aquaculture. Taken together, these results highlight the importance of monitoring the presence of potentially pathogenic strains in the natural Vibrio population of bivalve farming areas to reduce the risk to seafood consumers.
•Eight Harveyi subclades identified in the eastern Adriatic bivalve aquaculture.•Isolates of all Harveyi subclades displayed substantial genetic heterogeneity.•Typical V. harveyi virulence factors spread across all Harveyi clade species.•>60% of isolates displayed resistance to at least two antibiotic classes.•Monitoring of Vibrio spp. in the Adriatic bivalve aquaculture strongly recommended.
This research was conducted between March 2016 and March 2017 as the first study on the water and mussel quality in the coastal area of Dubrovnik (Croatia). Bacteriological analyses were conducted ...simultaneously on wild mussels (Mytilus galloprovincialis) tissue and their ambient water. The total numbers of heterotrophic bacteria, total coliforms, faecal coliforms and faecal streptococci in the water were determined. Concentrations of Escherichia coli were measured in mussel. The investigated stations are located in coastal water, which is particularly influenced by climatic and anthropogenic factors interacting at the boundaries between the land and freshwater and seawater environments. Seasonal variables, such as rainfall and riverine network characteristics, determine the delivery of bacteria (including E.coli) to coastal water. Direct methods of assessing the density of heterotrophic bacteria at all sampling stations showed the characteristics of an oligotrophic area. The abundance of faecal coliform bacteria was in all cases higher within mussel tissue than in the surrounding water where levels were sometimes undetectable. Maximum E. coli concentrations in mussels and seawater were regularly recorded through the summer, with the exceptions of April 2016 and January 2017 at two stations following precipitation. Minimum faecal coliform concentrations were recorded in the winter.
The initial colonization of bacteria and diatoms on a immersed artificial substrate and the development of diatom assemblages in relation to physico-chemical parameters were investigated on a weekly ...basis at one station in the marine Lake Mrtvo More, South Croatia, from April to October 2016. According to TRIX trophic index, lake showed different trophic character: (i) oligotrophic (at the beginning and the end of the study), (ii) mesotrophic (the end of June to mid-July), (iii)
eutrophic (the end of July to mid-September). Heterotrophic bacteria increased to peak abundances (69,268 cells cm–2) at the beginning of June when the diatoms abundances start to increase. The lake has high diatom species richness (285 diatom taxa within 72 genera), with the highest species diversity index in August. Among diatoms, adnate were the primary colonizers, particularly Cocconeis dirupta W.Gregory var. flexella (Janisch and Rabenhorst) Grunow and Cocconeis scutellum Ehrenberg var. scutellum, while motile taxa joined the fouling communities from July to September. This study showed close relationship
between diatom species composition and changes of physico-chemical parameters, particularly the nutrient concentrations.
This study sheds light on the influence of dissolved oxygen (DO) and temperature on fine-scale zooplankton vertical distribution in a seasonally stratified, enclosed marine lake on the Croatian ...island of Mljet. Unusually high precipitation during the spring and summer of 2014 caused extended hypoxia above the bottom in the autumn. Nauplii, copepodites, Oithona nana and appendicularians strictly avoided the layers of reduced DO. Copepods Oithona similis, Mesaiokeras hurei, oncaeids, and polychaeta trochophore comprised a community adapted to the hypoxic bottom layer (10%-14% dissolved oxygen). Oncaeids were the most abundant and were presented by two small species: Oncaea zernovi and the more numerous Monothula subtilis. Polychaeta larvae were extraordinary numerous, too. Proliferation of ocaeids and polychaeta larvae in hypoxic zone could be related to their ability to feed on detrital matter and associated microbial community. Contrary, low abundance of other copepod developmental stages were found in the whole water column suggesting that the high load of organic material and hypoxic events inhabited their reproduction. Our study has documented the dramatic impact of local environmental changes on the planktonic community, providing additional evidence that low oxygen has an effect on the distribution and abundance of zooplankton.
Abstract We evaluated the seasonal phosphorus (P) status of the plankton community in the Ombla Estuary (OE) by using its specific alkaline phosphatase activity (sAPA). Microphytoplankton (MICRO) ...indicated a substantially higher P deficiency than nanophytoplankton (NANO) and picoplankton (PICO). We found that the prolonged increase in the temperature of the surface estuarine water supported a notable growth of the dinoflagellate Prorocentrum spp. in late spring-early summer (June). In the summer (August), we found the maximum microphytoplankton sAPA (MICRO sAPA) (307.8 nmol μg C-1 h-1) in the surface water, in which (84%) dinoflagellates predominated within MICRO with the maximum alkaline phosphatase activity (APA) in all size fractions, including free enzymes. Persistently low discharge of Ombla during summer-early autumn caused a transition from phosphorus- to potentially nitrogen-limited MICRO in the surface water in early autumn (October). Nutrient stress disappeared in winter, in which a significant amount of dissolved orthosilicate, dissolved inorganic nitrogen (DIN), and soluble reactive phosphorus (SRP) enriched the estuary via maximal river discharge and inflow of nutrient-rich coastal waters. MICRO (coccolithophorids and diatoms) had very low APA (surface water) and quantitatively undetectable APA (bottom water) in the nutrient-rich water column in January. This study shows a more significant impact of nutrient concentrations on MICRO than other size classes of the plankton community. Because of the similarity in seasonal hydrological features, we assume that the general pattern of switching from P- to N-limitation of phytoplankton growth also occurs in other highly stratified estuaries along the coastal karst of the eastern Adriatic Sea during the lowest river discharges and groundwater activities in summer-early autumn before the rainy season. This study indicates that a common highly stratified estuary on the eastern Adriatic coast can serve as a natural laboratory to explore connections between nutrient limitations and phytoplankton successions.
An unusual phenomenon was recorded in the deep South Adriatic in February 2008: phytoplankton reached their maximum abundance in the aphotic zone. This was associated with strong downward flow ...induced by (1) surface cooling during bura (the region's cold, dry north wind) events and (2) ingression of more saline Eastern Mediterranean Water. Maximum abundance of microphytoplankton (MICRO) and nanophytoplankton (NANO), 1.4×104 and 6.8×105cellsL−1, respectively, was at 400m in the South Adriatic Pit. Diatoms dominated deep microphytoplankton samples. Atypically, autotrophic picoflagellates were found throughout the water column, with high abundance in the aphotic layers. Zooplankton did not show such dramatic deviations from their usual vertical distribution, but the abundance and relative contribution of some species between 200 and 800m layer were markedly different.
► High phytoplankton abundance in the aphotic zone is the evidence of deep convection. ► Convection affects nano- and microphytoplankton distribution the most. ► General pattern of zooplankton distribution is not affected by convection. ► The whole mixing layer should be sampled for real estimation of primary production.
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