Autumn Water Research Articles

Shifts in neritic copepod communities off the Basque coast (southeastern Bay of Biscay) between 1998 and 2015

Abstract The interannual variations in absolute and relative densities of copepods from the neritic waters of the southeastern Bay of Biscay and their relationship to climate teleconnections and local environmental factors were assessed using time series for the 1998–2015 period. Opposite patterns of variation of the absolute densities of spring Acartia clausi and Centropages typicusspecies vs. summer/autumn species, mainly Oncaea media, but also Ditrichocorycaeus anglicus, Oithona nana, Temora stylifera, and Oithona plumifera were detected. This type of opposite patterns were also observed between the relative densities of the spring A. clausi and summer Paracalanus parvus species. These opposite density patterns were positively correlated to all seasons North Atlantic Oscillation (NAO), spring East Atlantic pattern (EA), summer and autumn water temperature and summer chlorophyll a. They were negatively correlated to summer EA pattern, the winter and spring Atlantic Multidecadal Oscillation (AMO) and water temperature, and the upwelling index of all seasons. In these patterns of copepod variation two regime shifts were detected, one in 2008 towards an increase in the spring species and the other one in 2014–2015 towards an increase of summer species. This latter regime shift coincides in time with the abrupt community shifts predicted in the literature for 2014.

Hot, wet and rare: modelling the occupancy dynamics of the narrowly distributed Dixie Valley toad

Context Small population sizes and no possibility of metapopulation rescue put narrowly distributed endemic species under elevated risk of extinction from anthropogenic change. Desert spring wetlands host many endemic species that require aquatic habitat and are isolated by the surrounding xeric terrestrial habitat. Aims We sought to model the occupancy dynamics of the Dixie Valley toad (Anaxyrus williamsi), a recently described species endemic to a small desert spring wetland complex in Nevada, USA. Methods We divided the species’ range into 20 m × 20 m cells and surveyed for Dixie Valley toads at 60 cells during six primary periods from 2018 to 2021, following an occupancy study design. We analysed our survey data by using a multi-state dynamic occupancy model to estimate the probability of adult occurrence, colonisation, site survival, and larval occurrence and the relationship of each to environmental covariates. Key results The detection probabilities of adult and larval toads were affected by survey length and time of day. Adult Dixie Valley toads were widely distributed, with detections in 75% of surveyed cells at some point during the 3-year study, whereas larvae were observed only in 20% of cells during the study. Dixie Valley toad larvae were more likely to occur in cells far from spring heads with a high coverage of surface water, low emergent vegetation cover, and water temperatures between 20°C and 28°C. Adult toads were more likely to occur in cells with a greater coverage of surface water and water depth >10 cm. Cells with more emergent vegetation cover and surface water were more likely to be colonised by adult toads. Conclusions Our results showed that Dixie Valley toads are highly dependent on surface water in both spring and autumn. Adults and larvae require different environmental conditions, with larvae occurring farther from spring heads and in fewer cells. Implications Disturbances to the hydrology of the desert spring wetlands in Dixie Valley could threaten the persistence of this narrowly distributed toad.

Isotopic reconstruction of iron oxidation-reduction process based on an Archean Ocean analogue

The chemical composition and redox conditions of the Precambrian ocean are key factors for reconstructing the temporal evolution of atmospheric oxygen through time. In particular, the isotopic composition of iron are useful proxies for reconstructing paleo-ocean environments. Yet, respective processes and related signatures are poorly constrained, hindering the reconstruction of iron redox mechanisms in the Archean ocean. This study centers on Sihailongwan Lake, a stratified water body with a euxinic lower water column considered as an Archean ocean analogue. Results show that the anaerobic oxidation layer is so different from other similar lakes in which dissolved Fe oxidation is present in redoxcline layer. And the fractionation factor between ferrous Fe and iron hydroxide observed in nature water body of Sihailongwan Lake reaches to 2.6‰, which would benefit the production of the oxidations of BIF in sediment. By the spatial distribution of Fe isotope, the benthic water in autumn and the hypolimnetic anoxic water in spring has been identified as iron sulfide zone, where iron isotopic fractionation factor during iron sulfide formation is 1.16‰, accounting for partial scavenging of dissolved Fe(II) with an associated isotopic fractionation. However, pyrite in the sediment records the iron isotopic signal from the redoxcline but not in the iron sulfide or oxide zones of the water column. Above findings indicate that neither the iron isotope fractionation during partial transfer of ferrous iron to iron sulfide nor the partial oxidation of ferrous iron are recorded as pyrite in sedimentary rock. Importantly, the signal of Fe isotopic fractionation in water was archived in the suspended particulate matter and transferred into the sediment, rather than via ferrous iron directly deposited in the sediment. This study reveals that Fe isotopes from modern natural environments are useful proxies for reconstructing iron oxidation-reduction process during Earth's early history.

Sampling design optimization based on estimation accuracy and survey cost for fishery-independent surveys

Optimization of sampling design can reduce survey cost, ensure the accuracy of survey data, and get the maximum benefit of survey design. In this study, survey voyage was added into sampling survey design optimization as the evaluation index to quantify the survey cost. Computer simulation and resampling technique were used to simulate the survey plan. Simulation annealing algorithm was used to find the survey design with the shortest voyage. Based on the survey data collected from the bottom trawl survey conducted in the Haizhou Bay and its adjacent waters in spring and autumn from 2013 to 2015, Kriging interpolation was used to simulate the relative abundance distribution of Conger myriaster, Enedrias fangi, and Hexagrammos otakii in the bay in two seasons as their 'true' values. Resampling was conducted using stratified random sampling with different sample sizes for simulation study, and the abundance indices of each target species were estimated based on the simulated data. The relative bias (RB) was used to evaluate the accuracy of estimation of abundance index. The average survey cost and probability of budget overshoot (P) were used to measure the cost of survey. Integrated evaluation index (IEI) was developed to measure the survey cost and estimation accuracy comprehensively. The results showed RB values of all target species decreased with sample size. Different target species had different RB values with the same sample size. The optimal numbers of station determined by different evaluation indices were different. The optimal sample size determined by RB was relatively high, while the cost exceeded the budget of survey. The optimal sample size determined by probability of exceeding survey budget was relatively low but the accuracy and precision of estimation was low. IEI balanced the survey cost and estimation accuracy, and the optimal sample size defined by IEI was at intermediate level. The sampling design considering survey cost and route planning could not only determine the optimal sample size, but also record the potential station location, the survey sequence and the corresponding survey cost of simulations. According to the actual cost budget, the desired survey design with low cost and low RB of estimation from simulations could be chosen for fishery-independent surveys.

First Perceptions of Hydroperiod Mapping and Assessment of Shallow Waters in Coastal Landscapes by Drone-Based Monitoring Activities: A Remote-Sensing and GIS Approach

Beyond the importance of ponds for aquatic and terrestrial life, pond networks seem to be crucial to providing a vital spatial resource in response to global climate change for all migrating and spreading taxa. Additionally, ponds offer sustainable solutions to issues of concern in water management, such as nutrient retention, rainfall interception, or carbon sequestration. Although the ecological role of shallow waters seems clear, significant work must be performed to set future guidelines and actions towards their conservation. The main aims of the present study are to (i) georeference all small temporary wetlands within the Tyrrhenian central Italy coastal area, (ii) evaluate their hydroperiod, and (iii) calculate their surface size variability. We found 137 wetlands, 53 of which were temporary and contained listed habitats. Each wetland’s status was assessed in relation to land use and proximity to stressors (e.g., urban centres, railways, roads) while observing the relationship between pond occurrence, lithology, and permeability. Amongst the detected wetlands, we selected and monitored 21 temporary ponds (homogeneously distributed within the study area) for 12 months using images collected by the non-professional drone Parrot Bebop 2. All images were then acquired in ArcGIS to georeference all temporary ponds. The analysis confirmed that the majority of the surveyed ponds are in close proximity to roads and tracks, which might have significant impacts on the preservation of such fragile habitats. Moreover, despite the wide variability of hydroperiod duration, the greater part of the pools fill with water in autumn and dry in summer, in alignment with the Mediterranean climate. This preliminary study allowed for the creation of the first temporary ponds’ database, which is useful for monitoring their status in central Italy and planning further studies to assess eventual detrimental effects caused by human-mediated activities.

Global Connectivity of Southern Ocean Ecosystems

Southern Ocean ecosystems are globally important. Processes in the Antarctic atmosphere, cryosphere, and the Southern Ocean directly influence global atmospheric and oceanic systems. Southern Ocean biogeochemistry has also been shown to have global importance. In contrast, ocean ecological processes are often seen as largely separate from the rest of the global system. In this paper, we consider the degree of ecological connectivity at different trophic levels, linking Southern Ocean ecosystems with the global ocean, and their importance not only for the regional ecosystem but also the wider Earth system. We also consider the human system connections, including the role of Southern Ocean ecosystems in supporting society, culture, and economy in many nations, influencing public and political views and hence policy. Rather than Southern Ocean ecosystems being defined by barriers at particular oceanic fronts, ecological changes are gradual due to cross-front exchanges involving oceanographic processes and organism movement. Millions of seabirds and hundreds of thousands of cetaceans move north out of polar waters in the austral autumn interacting in food webs across the Southern Hemisphere, and a few species cross the equator. A number of species migrate into the east and west ocean-basin boundary current and continental shelf regions of the major southern continents. Human travel in and out of the Southern Ocean region includes fisheries, tourism, and scientific vessels in all ocean sectors. These operations arise from many nations, particularly in the Northern Hemisphere, and are important in local communities as well as national economic, scientific, and political activities. As a result of the extensive connectivity, future changes in Southern Ocean ecosystems will have consequences throughout the Earth system, affecting ecosystem services with socio-economic impacts throughout the world. The high level of connectivity also means that changes and policy decisions in marine ecosystems outside the Southern Ocean have consequences for ecosystems south of the Antarctic Polar Front. Knowledge of Southern Ocean ecosystems and their global connectivity is critical for interpreting current change, projecting future change impacts, and identifying integrated strategies for conserving and managing both the Southern Ocean and the broader Earth system.

Holm oak fecundity does not acclimate to a drier world.

Climate change might impact tree fecundity by altering the relative influences of meteorological and physiological drivers, and by modifying resource investment in reproduction. Using a 13-yr monitoring of Quercus ilex reproduction in a rainfall exclusion experiment, we analysed the interactive effects of long-term increased aridity and other environmental drivers on the inter-annual variation of fecundity (male flower biomass, number of initiated and mature fruits). Summer-autumn water stress was the main driver of fruit abortion during fruit growth. Rainfall exclusion treatment strongly reduced the number of initiated and mature fruits, even in masting years, and did not increase fruit tolerance to severe drought. Conversely, the relative contribution of the meteorological and physiological drivers, and the inter-annual variability of fruit production were not modified by rainfall exclusion. Rather than inducing an acclimation of tree fecundity to water limitation, increased aridity impacted it negatively through both lower fruit initiation due to changes in resource allocation, and more severe water and resource limitations during fruit growth. Long-term increased aridity affected tree reproduction beyond what is expected from the current response to inter-annual drought variations, suggesting that natural regeneration of holm oak forest could be jeopardised in the future.

Remote Sensing-Based Analysis of Spatial and Temporal Water Colour Variations in Baiyangdian Lake after the Establishment of the Xiong’an New Area

The Forel-Ule Index (FUI) is an important parameter that can be calculated from optical remote sensing data to assess water quality based on water colour. Using Sentinel-2 images from April to November within the 2016–2020 period coupled with the Google Earth Engine Platform, we calculated FUI to analyse the spatial distribution, seasonal variations, and inter-annual variations of water colour in Baiyangdian Lake in the Xiong’an New Area established on 1 April 2017. The lake was divided into seven sub-regions, A–G; subsequently, high and low FUI values were observed in the south and north, respectively. Additionally, the mean FUI values of G and F zones in the south were 11.9 and 12.7, respectively, whereas those for the A, B, C, D, and E zones in the north were 10.5, 9.8, 10.4, 11.1, 11.2, respectively. The seasonal variations in the Baiyangdian Lake and seven sub-regions were consistent, with turbid water in spring and autumn, and clear water in summer. Inter-annual variations analyses for 2016–2020 indicated that the zone of A became progressively turbid, whereas the B, C, D, E, F, and G zones exhibited slow and gradually decreasing trends. Our findings suggest that the overall water quality of Baiyangdian Lake may be better, which may be related to the governance policies of the region.

Numerical Study on the Expansion and Variation of Changjiang Diluted Water in Summer and Autumn

In view of the expansion and directional change mechanisms of Yangtze River water diluted with sea water in the shelf region (also known as Changjiang diluted water [CDW]) during summer and autumn, a three-dimensional hydrodynamic model of the Yangtze River Estuary (YRE) and its adjacent waters was established based on the Finite Volume Community Ocean Model (FVCOM). Compared with the measured data, the model accurately simulates the hydrodynamic characteristics of the YRE. On that basis, the influence of the expansion patterns of the CDW in both summer and autumn was studied. It was found that, in 2019, the CDW expanded to the northeast in the summer and to the southeast in the autumn, and that the route of the CDW is mainly controlled by the wind, not the runoff. Current seasonal winds also change the transportation route of the CDW by affecting its hydrodynamic field. Typhoons are frequent in both summer and autumn, causing abnormalities in both the transportation route and expansion of the CDW. During a typhoon, a large amount of the CDW is transported in a continuous and abnormal manner, accelerating the path turning of the CDW. This paper enhances the existing theoretical research of the CDW and provides a reference with respect to the expansion of diluted water all over the world.

Changes in population depth distribution and oxygen stratification are involved in the current low condition of the eastern Baltic Sea cod (<i>Gadus morhua</i>)

Abstract. During the past 20 years, hypoxic areas have expanded rapidly in the Baltic Sea, which has become one of the largest marine “dead zones” in the world. At the same time, the most important commercial fish population of the region, the eastern Baltic cod, has experienced a drastic reduction in mean body condition, but the processes behind the relation between deoxygenation and condition remain elusive. Here we use extensive long-term monitoring data on cod biology and distribution as well as on hydrological variations to investigate the processes that relate deoxygenation and cod condition during the autumn season. Our results show that the depth distribution of cod has increased during the past 4 decades at the same time of the expansion, and shallowing, of waters with oxygen concentrations detrimental to cod performance. This has resulted in a progressively increasing spatial overlap between the cod population and low-oxygenated waters after the mid-1990s. This spatial overlap and the actual oxygen concentration experienced by cod therein statistically explained a large proportion of the changes in cod condition over the years. These results complement previous analyses on fish otolith microchemistry that also revealed that since the mid-1990s, cod individuals with low condition were exposed to low-oxygen waters during their life. This study helps to shed light on the processes that have led to a decline of the eastern Baltic cod body condition, which can aid the management of this population currently in distress. Further studies should focus on understanding why the cod population has moved to deeper waters in autumn and on analyzing the overlap with low-oxygen waters in other seasons to quantify the potential effects of the variations in physical properties on cod biology throughout the year.

Солевой состав вод Бурейского водохранилища

Significance. Significance of the work lies in the need to study the salt composition of the Bureya reservoir water from the beginning of filling, which is important for predicting the water quality of reservoirs in the Amur basin that are to be designed. The objective is to evaluate content of main ions in the water of the Bureya reservoir, and to identify factors determining their spatial and temporal variability. Methods. Hydro/chemical observations in the Bureya reservoir were made from 2003 to 2008 at five plots covering the major part of the water area. Water samples were taken from the surface, middle, and near-bottom horizons in winter, summer, and fall. Water samples were taken from the surface in January, July 2019 in the landslide area and in July 2020 at eight sites in the water body between the landslide and the hydropower dam. Analytical work was carried out according to the methods accepted in hydrochemistry. Materials of the Amur Basin Water Administration for 2005-2019 were used in the work. Results. The content of the main ions in the water of the reservoir during the filling period and the following years was assessed. The dominance of calcium ion among cations and hydrocarbonate ion among anions in the salt composition of water was shown. It is established that the emergence of oxygen deficit in water due to the increased area of flooded forests and reduced water inflow caused the absence of sulfate ion in the reservoir water in autumn and winter 2005. It is shown that staged filling of the reservoir contributed to gradual smoothing of differences in the content of major ions and stabilization of the salt composition of water in the sixth year of filling. In current conditions the content of sodium and potassium ions and chloride ions in the water area of the reservoir is relatively evenly distributed, the highest values of concentrations of calcium and magnesium hydrogen carbonates are observed near the mouth of the Tyrma River. The effect of a large landslide on the content of major ions in the water in 2019-2020 is considered.

Harp seal body condition and trophic interactions with prey in Norwegian high Arctic waters in early autumn

In September 2016, a marine ecosystem survey covered all trophic levels from phytoplankton to seals in the Arctic Ocean to the west and north of Svalbard. At the ice edge, 26 harp seals were sampled to assess whether recent environmental changes had affected their diets and body condition by comparing our current results with previous investigations conducted 2–3 decades ago in the northern Barents Sea, when the ice edge was located much further south. Our results suggest that the body condition was slightly but significantly lower for one year and older seals in 2016 compared with seals sampled in the early 1990s. Furthermore, we confirmed previous findings that polar cod (Boreogadus saida) and the pelagic hyperiid amphipod Themisto libellula still dominate the seal diet. One consequence of current ice edge localisation north of Svalbard is that the water depth underneath is now 500 m and deeper, which probably explains the absence of bottom associated species, and the presence of species such as Atlantic cod (Gadus morhua) and blue whiting (Micromesistius poutassou) as alternative species in addition to polar cod and T. libellula in the seal diets. Stable isotope data also suggest possible long-term importance in the seal diet of T. libellula and of low trophic level benthopelagic prey such as the squid Gonatus fabricii over mid-trophic level pelagic fishes, but with a strong component of small, benthopelagic fish such as blue whiting. The long-term importance of pelagic crustaceans was also suggested from the fatty acid analyses. Assessment of the abundance of prey showed that T. libellula was by far the most abundant prey species in the upper water layers, followed by krill (mainly Thysanoessa inermis), Atlantic cod and polar cod. Prey-preference analyses indicated that polar cod was the most preferred prey species for the seals.

Occurrence and Fate of Steroid Estrogens in a Chinese Typical Concentrated Dairy Farm and Slurry Irrigated Soil.

Animal husbandry is the second largest source of steroid estrogen (SE) pollutants in the environment, and it is significant to investigate the occurrence and fate of SEs discharged from concentrated animal feeding operations. In this research, with a Chinese typical concentrated dairy farm as the object, the concentrations of SEs (E1, 17α-E2, 17β-E2, E3, and E1-S3) in slurry, lagoon water, and slurry-irrigated soil samples in summer, autumn, and winter were determined. The total concentrations of SEs (mainly E1, 17α-E2, and 17β-E2) in slurry were very high in the range of 263.1-2475.08 ng·L-1. In the lagoon water, the removal efficiencies of the aerobic tank could reach up to 89.53%, with significant fluctuation in different seasons. In the slurry-irrigated soil, the maximum concentrations of SEs in the topsoil and subsoil were 21.54 ng·g-1 to 6.82 g·g-1, respectively. Most of the SEs tended to transport downward and accumulate in the soil accompanied with the complex mutual conversion. Correlations and hierarchical clustering analysis showed a variety of intertransformation among SEs, and the concentrations of SEs were correlated with various physicochemical indexes, such as TN and NO3--N of the slurry, chemical oxygen demand of the lagoon water, and the heavy metals of soil. In addition, 17β-estradiol equivalency assessment and risk quotients indicated that the slurry irrigation and discharge of the lagoon water would cause potential estrogenic risks to the environment. Consequently, reasonable slurry irrigation and lagoon water discharge are essential to efficiently control SE pollution in the environment.

First baseline for bioenergetic biomarkers in Cnesterodon decemmaculatus as test organism in ecotoxicological studies

Cnesterodon decemmaculatus is a Neotropical teleost fish frequently used in ecotoxicological evaluations, whose biology has been thoroughly studied. Although there is considerable information on its response to different toxicants, no range of reference values has been so far established for the different biological parameters proposed as biomarkers of effect or exposure. Moreover, no study has yet examined the possible influence of the metabolic status of the exposed animals on their response to toxic stress. Therefore, the aim of this work was to provide a first baseline for a set of bioenergetic biomarkers in C. decemmaculatus adults exposed to a control medium under previously standardized conditions, and to assess their possible intrinsic seasonal variability. The responses of the biomarkers obtained from the controls were contrasted with those from the reference toxicant (Cadmio-Cd) and receiving waters (surface waters of the Reconquista River RR, Buenos Aires Province, Argentina). We conducted four 12-day assays (one in each season) of exposure to control media, (reconstituted moderate hard water, MHW) and two assays of exposure to Cd in MHW and surface river water (RR) in both summer and autumn. The variables recorded were: Food intake (In), fecal production (F), specific assimilation (A) and cumulative mortality, oxygen extraction efficiency (OEE), specific metabolic rate (SMR), ammonia excretion (N), ammonia quotient (AQ) and scope for growth (SFG). The seasonal variation shown by some physiological parameters, points to the need for establishing a baseline obtained from standardized media, preferably on a seasonal basis. Moreover, SFG and A appeared as the most sensitive biomarkers, emphasizing the importance to consider the metabolic status of the test organisms for the appropriate interpretation of results from ecotoxicological studies performed under controlled experimental conditions. The obtained results provide useful information on C. decemmaculatus as model species in ecotoxicological bioassays involving biomarkers of early effect.

Evaluation of the effects of climate change on thermal stratification of reservoirs

Many water needs are provided by reservoirs. Increasing water retention time in reservoirs leads to thermal stratification and, consequently, affects the water quality of reservoirs. This situation can be exacerbated under climate change. The objective of this study is to assess the climate change effect on thermal stratification of the Aidoghmoush reservoir (East Azarbaijan, Iran) for baseline (1987–2000) and climate change (2026–2039) periods. For this purpose, future temperature and precipitation were estimated by HadCM3 under the A2 emission scenario, and inflow of the reservoir in 2026–2039 was simulated by IHACRES. Results show that the average annual reservoir inflow in climate change decreases about 1% relative to the baseline. In addition, CE-QUAL-W2 was calibrated and used to simulate the thermal stratification of the reservoir. Results show that on average, the surface and bottom water temperatures of the reservoir during the climate change period respectively increase by 1.69 and 1.24 °C compared to those of the baseline. The maximum and minimum surface water temperatures under climate change are respectively 0.14–3.98 °C higher than those in the baseline. Under climate change, thermal stratification mainly occurs in shallow water of the reservoir in spring and summer, but it is extended to deeper water in autumn. In winter, however, complete mixing may occur.

Different Summer and Autumn Water Deficit Affect the Floral Differentiation and Flower Bud Growth in Apricot (Prunus armeniaca L.)

In deciduous fruit species, floral bud initiation, differentiation and organogenesis take place during the summer–autumn season that precedes anthesis. Among factors able to modify the regularity of these processes, water availability represents a crucial aspect. This investigation aimed to assess the influence of different summer and autumn water deficit and re-watering treatments on floral morphogenesis, xylem vessel differentiation and quality of flower buds. Trials were carried out on two-year-old potted apricot trees (cv. ‘Portici’) which were submitted to different regimes: (i) fully irrigated plants; (ii) stressed plants in June (S1), July (S2) and October (S3) followed to re-watering. Midday stem water potential was used to determine water status, and leaf gas exchanges were measured during trials. Histological analyses on floral differentiation, xylem progression within flower buds and biological observations were carried out. Both summer water stress periods affected the floral differentiation leading to a temporary shutdown. The S1 trees were able to recover the development of meristematic apices while S2 had a strong delay. All drought treatments caused a slower xylem progression, variations in bud size, blooming entity and flower anomalies. Results particularly highlights the importance of water availability also in early autumn.

Research Expeditions Performed by Marine Hydrophysical Institute in the Sivash Bay Waters in Spring and Autumn, 2018

Research Expeditions Performed by Marine Hydrophysical Institute in the Sivash Bay Waters in Spring and Autumn, 2018

Spatial Differentiation and Multiannual Dynamics of Water Conductivity in Lakes of the Suwałki Landscape Park

Water conductivity in 23 lakes of the Suwałki Landscape Park (SLP) was tested in the years 2012–2014. Conductivity profiles were made at the deepest place every two months between spring and autumn water mixing. The collected measurement data, supplemented with historical data, were used to identify factors that shape the spatial variability of water conductivity and to reconstruct its multiannual changes. The range of variability of the mean conductivity of surface water of the SLP lakes ranged from 178 to 522 µS cm−1. The strong negative relationship between conductivity and lake elevation (R = 0.816, p < 0.000) suggests that in the territorially compact complex of the SLP lakes, conductivity is a consequence of the location of the lake in the catchment, which, in turn, affects the structure of its water supply. However, the physical and environmental parameters of the catchment and morphometric parameters of lake basins are of secondary importance. In dimictic lakes and in those showing signs of “spring meromixis”, the magnitude of conductivity differences in the water column is determined by the lakes’ susceptibility to wind mixing expressed by the exposure index value. The developed climate models have also shown that conductivity, an indicator of water quality, is very sensitive to climate change. The analysis of the reconstructed chronological conductivity sequences shows that the deterioration of the quality of the waters of the SLP lakes first occurred at the beginning of the 21st century and a clear increasing trend has been maintained over the last decade.

Diverse effects of climate at different times on grassland phenology in mid-latitude of the Northern Hemisphere

• SOS/EOS/LGS has significantly advanced/delayed/prolonged in dominated areas. • Temperature and precipitation showed stronger control on SOS and EOS, respectively. • The acting time of water condition on SOS is earlier and longer than temperature. • EOS primarily depends on spring thermal conditions and autumn water availability. Studying grassland phenology and its relationships to climate would deepen our understanding of vegetation-air interactions under global climate change. To date, however, our knowledge of the responses of grassland phenology to climatic factors is still limited at the continental scale. In this study, we retrieved the start (SOS) and end (EOS) of the growing season for mid-latitude (30°N–55°N) grasslands of the Northern Hemisphere during 1981–2014, and investigated their relations with previous temperature, rainfall, and snowfall (only for SOS) through trends analysis and time window analysis. Results illustrated a predominant significant advancing/delaying trend of SOS/EOS in 23.2%/20.5% of the study region. They jointly resulted in a primarily significant prolongation trend of growing season length in 22.7% of the study region. Next, a dominated negative correlation between air temperature/rainfall and SOS was found in 62.4%/57.6% of areas. Snowfall showed converse effects (positive/negative) among different grasslands. The time window opening date for air temperature to start to affect SOS was identified as the day 1–90 before the multi-year average SOS in 76.1% of areas, while the time window opening date for the effect of rainfall/snowfall on SOS was relatively evenly distributed between the 1st and 180th day before the multi-year average SOS. EOS was found to be significantly negatively/positively correlated with air temperature/precipitation in 74.8%/83.7% of areas. The time window opening date for the effect of air temperature on EOS was identified as the 90–180th day before the multi-year average EOS in 66.9% of areas, while the time window opening date for the effect of precipitation on EOS was mainly concentrated on the 60-120th day before the multi-year average EOS in 51.5% of areas. Overall, this study highlights the distinctly different time windows for the thermal-moisture effects on grassland vegetation phenology and this should be considered when establishing process-based phenological models.

Decoupling Between Phytoplankton Growth and Microzooplankton Grazing Enhances Productivity in Subantarctic Waters on Campbell Plateau, Southeast of New Zealand

AbstractThe Subantarctic zone is one of the largest High‐Nutrient Low‐Chlorophyll zones of the Southern Ocean. Despite widespread iron limitation, phytoplankton accumulation (chlorophyll a (chla) > 0.3 mg m−3) often occurs near islands and bathymetric features such as on the Campbell Plateau, southeast of New Zealand. To investigate the processes responsible for localized increases in chla commonly observed by satellites, we characterized phytoplankton biomass structure, production, and microzooplankton grazing on Campbell Plateau and surrounding waters in austral autumn (March 2017). Chla on the plateau tended to be higher, more variable (0.52 ± 0.38 mg chla m−3, mean ± standard deviation), and characterized by larger phytoplankton forms (22 ± 27%chla > 20 μm) than surrounding waters (0.29 ± 0.12 mg chla m−3, 5 ± 2%). The increased contribution of diatoms, together with higher photosystem II maximum photochemical efficiency (Fv/Fm = 0.45 ± 0.05) and lower effective absorption cross‐section (σPSII = 774 ± 90 Å RCII−1) on the plateau, suggests an alleviation of iron stress relative to surrounding waters (Fv/Fm = 0.37 ± 0.04, σPSII = 974 ± 89 Å RCII−1). Phytoplankton growth (μ0 = 0.42 ± 0.20 day−1) and production rates (6.1 ± 3.2 mg C m−3 day−1) were also higher compared to surrounding waters (0.27 ± 0.04 day−1, 3.5 ± 1.9 mg C m−3 day−1). While microzooplankton grazing (g = 0.28 ± 0.18 day−1) balanced phytoplankton growth off the plateau (g:μ0 = 1.13 ± 0.18), the imbalance observed on Campbell Plateau (g = 0.25 ± 0.25 day−1) allowed a substantial proportion of primary production to escape microzooplankton grazing control (g:μ0 = 0.48 ± 0.31). Overall, the degree of coupling tended to decrease with the depth of the mixed layer (R2 > 0.6, p < 0.001). We hypothesize that the entrainment of deeper water into the mixed layer regulates the onset and fate of the autumn bloom by altering nutrient supply and microzooplankton grazing pressure.