Total Zooplankton Biomass Research Articles

Winter Zooplankton in a Small Arctic Lake: Abundance and Vertical Distribution

Zooplankton assemblages are of great importance in aquatic food webs because they link lower (microplankton) and higher trophic levels (top predators). Small water bodies in the Arctic regions of Russia are less studied in winter because of severe ice conditions. For this reason, we analyzed the winter zooplankton community in Lake Kulonga (western coast of Kola Bay, Barents Sea). A total of 9 taxa were found in the samples. The total abundance varied from 200 to 1320 ind. m−3, averaging 705 ind. m−3. The total zooplankton biomass was 1.8–72.8 mg of wet mass m−3 with an average of 30 mg m−3. These parameters were lower than in other Russian Arctic and sub-arctic lakes in summer. Old copepodites of Cyclops spp. dominated the zooplankton community at deep-water stations in terms of the total abundance consisting of 24–33%. The copepod Macrocyclops albidus prevailed in terms of the total zooplankton biomass comprising 30–33% at deep-water stations while Cyclops scutifer and copepodites Cyclops spp. had the highest biomass at shallow water stations. Vertical distribution demonstrated different patterns at neighboring stations, probably as a result of differences in the density of fish predators.

Current state and long-term changes in the mesozooplankton community of the Ukrainian and Georgian parts of the Black Sea as indicators of its ecological status

Water quality assessment is a key task of any measures in the field of water use, environmental management and protection. Thus, it is necessary to conduct systematic monitoring to assess the ecological state in the marine waters and to develop a strategy for its recovery. Anthropogenic impacts of various types leading to eutrophication and pollution of the Black Sea are changing the main characteristics of all components of the aquatic ecosystem. Zooplankton plays a key role in the pelagic food web. This article presents the results of the research on the state of zooplankton which was conducted during the Ukrainian-Georgian expedition in the framework of the international project “Emblas-plus” during 2016, 2017 and 2019. The ecological quality class of the investigated Black Sea waters was determined by the zooplankton integrated index (IZI). In Ukrainian waters in total, 49 taxa of zooplankton were registered. In 2017 taxonomic composition of zooplankton was more diverse (36 taxa in 2016, 35 in 2019). In Georgian waters in 2019, 40 taxa of mesozooplankton were registered, most of them are widespread forms in the Black Sea. The most diverse group is Crustacea (Cladocera and Copepoda).The most prevalent crustaceans were the eurythermic species, an important representative of forage zooplankton – Acartia (Acartiura) clausi Giesbrecht, 1889 and two thermophilic species of Calanoida – Acartia (Acanthacartia) tonsa Dana, 1849 and Centropages ponticus Karavaev, 1895. In Ukrainian waters average abundance and biomass of zooplankton in 2019 was similar to those in 2016 and much higher than in 2017. Dominant taxa and spatial distribution of zooplankton in 2019 were similar to those in 2016 and 2017. Average biomass of forage zooplankton in 2019 was approximately 10 times higher than in 2016 and 2017. In the Georgian coastal waters annual average abundance and biomass of mesozooplankton decreased, due to a sharp decline in the abundance of Noctiluca scintillans (Macartney) Kofoid & Swezy, 1921. It was found that at most of the stations the dominant role in the formation of zooplankton biomass was played by the organisms of forage zooplankton. By the IZI index, the subdivision Northwestern Black Sea Bays had “Good”, “Poor” and “Moderate” water quality in spring, summer and autumn respectively. The subdivision’s deepwater shelf and shallow shelf had “Bad” water quality. The subdivision Danube-Dnieper interfluve coastal waters had “High” water quality. The Danube Avandelta area had “High”, “Good” and “High” water quality in spring, summer and autumn respectively. The best ecological class status was in 2019 and the worst – in 2016. The main tendencies in changes in the mesozooplankton community in the Black Sea are decrease in the percentage of N. scintillans in the total zooplankton biomass and increase in the percentage of Copepoda. Those tendencies indicate decrease in the pressure of the negative eutrophication factor and show positive changes in the forage base of commercial planktophagous fish and the ecological status of the Black Sea waters.

Zooplankton variability in the Strait of Georgia, Canada, and relationships with the marine survivals of Chinook and Coho salmon.

The Strait of Georgia, Canada, has complex interactions among natural and human pressures that confound understanding of changes in this system. We report on the interannual variability in biomass of 12 zooplankton taxonomic groups in the deep (bottom depths greater than 50 m) central and northern Strait of Georgia from 1996 to 2018, and their relationships with 10 physical variables. Total zooplankton biomass was dominated (76%) by large-sized crustaceans (euphausiids, large and medium size calanoid copepods, amphipods). The annual anomaly of total zooplankton biomass was highest in the late 1990s, lowest in the mid-2000s, and generally above its climatological (1996-2010) average after 2011, although many individual groups had different patterns. Two latent trends (derived from dynamic factor analyses) described the variability of annual biomass anomalies underlying all zooplankton groups: a U-shaped trend with its minimum in the mid-2000s, and a declining trend from 2001 to 2011. Two latent trends also described the physical variables. The variability represented by these four latent trends clustered into two periods: 1996-2006, with generally declining zooplankton biomass and increasing salinities, and 2007-2018, with increasing zooplankton biomass and decreasing salinities. ARIMA modelling showed sea surface salinity at Entrance Island in the middle Strait of Georgia, the Pacific Decadal Oscillation, and the peak date of the spring phytoplankton bloom were significantly related to the two latent zooplankton trends. ARIMA models comparing zooplankton and physical variables with the marine survivals of four salmon populations which enter the Strait as juveniles (Chinook: Cowichan River, Puntledge River, Harrison River; Coho: Big Qualicum River) all included zooplankton groups consistent with known salmon prey; prominent among the physical variables were sea surface salinity and variables representing the flow from the Fraser River. These regressions explained (adjR2) 38 to 85% of the annual variability in marine survival rates of these salmon populations over the study time period. Although sea temperature was important in some relationships between zooplankton biomass and salmon marine survival, salinity was a more frequent and more important variable, consistent with its influence on the hydrodynamics of the Strait of Georgia system.

Багаторічні зміни біомаси Noctiluca scintillans ((Macartney) Kofoid & Swezy, 1921, Dinophyceae, Noctilucales) в Одеському та Дунайському районах Чорного моря як показника якості водного середовища

Water quality assessment is a key task of any measures in the field of water use, environmental managementand environmental protection. Zooplankton organisms are sensitive to changes in the environment and can be usedas indicators of the ecological status of aquatic ecosystems. One such organism is the heterotrophic dinoflagellateNoctiluca scintillans (Macartney) Kofoid & Swezy, 1921, Dinophyceae, Noctilucales. N. scintillans is a componentof the mesozooplankton of the Black Sea, which is not part of the forage base of planktophagous fish and is characterizedby a short life cycle. Due to this, it has an advantage in eutrophic conditions and can be used as an indicator of increasingtrophic levels of the reservoir. To determine the quality of the studied waters of the Black Sea was used the biomassindex of N. scintillans (%) of the total biomass of zooplankton. In the period from the 1970s (hypertrophication period)to 2010, the% of N. scintillans biomass from the total biomass of zooplankton in the Black Sea decreased annually from60% to 10% in Odessa region (Dnipro-Bug region) and from 90 % to 15% in the Danube region. Such changes indicatea decrease in the impact of the negative eutrophication factor and indicate positive changes in the feed base of industrialfish planktophages and the ecological status of the Black Sea. Ecological quality status (GES/NotGES) was determinedin the following regions of the Black Sea: the north-western Gulf of the Black Sea, the deep-water shelf, the shelf areaand the Danube Avandelta. “Good” ecological status (GES) % N. scintillans of total zooplankton biomass was observedin all research areas of the north-western part of the Black Sea, as the percentage of N. scintillans of total biomass didnot exceed 30%.

Interactive effects of bitterling fish and mussel on the community structures of plankton and benthic macroinvertebrates

放养河蚌，提高水体透明度以促进沉水植物生长，是湖泊生态修复中的常用手段之一.而小型杂食性鱼类鳑鲏依赖河蚌繁殖，河蚌放养可能会促进鳑鲏种群的发展；而鳑鲏与河蚌交互作用对水生态系统的影响仍研究较少.于2018年11—12月通过原位受控实验，设置对照组、河蚌组、鳑鲏组和河蚌+鳑鲏组，研究了鳑鲏（大鳍鱊Acheilognathus macropterus）与河蚌（背角无齿蚌Sinanodonta woodiana）对水质、浮游生物和底栖生物的影响.结果表明：鳑鲏显著增加了上覆水总氮、总磷、悬浮质和叶绿素a（Chl.a）浓度；但并未显著影响无机悬浮质的浓度，说明鳑鲏对沉积物的扰动作用较弱.此外，鳑鲏显著增加了浮游植物生物量和蓝藻的比例.浮游动物总生物量并未受鳑鲏的影响，但显著增加了浮游动物的丰度，其中以轮虫为主.鳑鲏组的底栖动物总丰度（主要为水丝蚓）显著高于对照组，说明鳑鲏对水丝蚓的摄食压力较低.鳑鲏与河蚌交互作用对总磷、Chl.a、浮游植物和浮游动物丰度具有显著影响：具体表现为河蚌虽然抵消了部分鳑鲏对水质（如氮、磷和悬浮质浓度，以及Chl.a浓度和浮游植物生物量等）的负面影响，但其对浮游动物和水丝蚓生物量的影响不显著.在湖泊生态修复和管理中，需要关注和重视鳑鲏等此类小型杂食性鱼类对水生态系统可能产生的负面影响，通过物理（网簖等）或生物（放养肉食性鱼类）等方式将其控制在较低密度水平，从而降低这些鱼类对水质、浮游生物和底栖生物群落可能产生的负面影响，维持生态修复效果的长效与稳定运行.;In restoration of shallow eutrophic lakes based on recovery of submerged macrophytes, stocking of the filter-feeding mussels has been commonly used for improving water clarity, thereby enhancing the growth of submerged macrophytes. However, the small omnivorous bitterling fish has to rely on mussels for breeding. Thus, the increased abundance of mussels may enhance the population development of bitterling. Nevertheless, the effects of bitterling (Acheilognathus macropterus) and their host mussels (Sinanodonta woodiana) on the aquatic ecosystems are, so far, not well-studied. We conducted a mesocosm experiment to study the effects of bitterling, mussels, and their interactions on water quality and the community structures of phytoplankton, zooplankton and benthic macroinvertebrates, respectively. Our results showed that bitterling did not substantially affect the concentration of inorganic suspended solids (ISS) indicating weak effects on sediment resuspension. However, the concentrations of total nitrogen and total phosphorus were significantly enhanced by bitterling. Consequently, both the abundance and biomass of phytoplankton were enhanced in the bitterling-only mesocosms, and cyanobacteria dominated the phytoplankton community at the end of the experiment. However, rotifers dominated the zooplankton community leading to a higher abundance of zooplankton in the bitterling-only treatment, while the total biomass of zooplankton did not differ significantly between the control and bitterling-only treatments. Moreover, bitterling significantly increased the total abundance of benthic macroinvertebrates compared with the controls, while the biomass of macroinvertebrates was not pronouncedly affected. The presence of mussels significantly eliminated the negative effects induced by bitterling, such as reducing the concentrations of nutrients, suspended solids and chlorophyll-a and phytoplankton biomass. However, the biomass of zooplankton and benthic macroinvertebrates were not substantially affected by the presence of mussels. Our results suggest that bitterling alone can induce significantly negative effects on water quality though mussels can eliminate part of these impacts under our experimental bitterling-to-mussel biomass ratio. Therefore, in the restoration and management of shallow lakes, more attention should be paid on the effects of small omnivorous fish, like bitterling, on aquatic ecosystems. The biomass of these small omnivores should be controlled at a relatively low level by physical (e.g., trap net) or biological (e.g., stocking of effective piscivorous fish) methods to minimize the negative effects of the fish on lake ecosystems.

Продуктивность зоопланктона в прибрежной зоне южной части Баренцева моря в весенний период

Представлены результаты анализа состояния зоопланктонного сообщества в южной части Баренцева моря. Пробы отобраны в ходе экспедиции НИС «Дальние Зеленцы» в мае 2016 г. Гидрологические условия были типичными для мурманской прибрежной водной массы в этот сезон. Всего обнаружено 47 таксонов зоопланктона. Их количество колебалась по станциям от 18 до 29. Копеподы были доминирующей группой в зоопланктоне. Наиболее часто встречались Calanus finmarchicus, Metridia longa, Metridia lucens, Microcalanus spp., Oithona atlantica, Oithona similis, Pseudocalanus spp., науплии и яйца копепод, а также кладоцеры Evadne nordmanni, личинки иглокожих, полихет, щетинкочелюстные Parasagitta elegans, ювенильные стадии эвфаузиид рода Thysanoessa. В составе популяций массовых видов копепод Pseudocalanus spp. и Oithona similis преобладали младшие возрастные группы, что свидетельствовало о продолжающемся их размножении. Суммарная численность зоопланктона варьировала от 748 до 6576 экз.·м−3, составляя в среднем 3012. Общая биомасса колебалась от 17 до 157 мг сухой массы·м−3, средняя величина равнялась 83. Полученные величи́ны сопоставимы с данными июля 2008 г. и превышают показатели августа 2007 г., что авторы связали с разными сезонами отбора проб и с различающимися гидрологическими условиями. Суточная продукция зоопланктона колебалась в диапазоне 0,49–4,04 мг сухой массы·м−3 при средней величине (2,17 ± 0,17), что примерно в 2 раза выше, чем средние показатели для мурманских прибрежных вод в летний период. Вероятно, эти различия связаны с более высокой концентрацией фитопланктона в весеннее время. Суммарная величина запаса зоопланктона в исследуемой акватории (25,8 тыс. км²) оценена в 425 тыс. т сухой массы. Кластерный анализ выявил наличие четырёх групп станций, которые различались соотношением Calanus finmarchicus, Copepoda nauplii, Oithona similis, личинок иглокожих и аппендикулярий Fritillaria borealis. Пространственную изменчивость численности зоопланктона определяли местоположение станций (широта, долгота, глубина), а также температура придонного слоя и средняя солёность вод на станции.

Impact of Nutrients, Temperatures, and a Heat Wave on Zooplankton Community Structure: An Experimental Approach

Shallow lakes are globally the most numerous water bodies and are sensitive to external perturbations, including eutrophication and climate change, which threaten their functioning. Extreme events, such as heat waves (HWs), are expected to become more frequent with global warming. To elucidate the effects of nutrients, warming, and HWs on zooplankton community structure, we conducted an experiment in 24 flow-through mesocosms (1.9 m in diameter, 1.0 m deep) imitating shallow lakes. The mesocosms have two nutrient levels (high (HN) and low (LN)) crossed with three temperature scenarios based on the Intergovernmental Panel on Climate Change (IPCC) projections of likely warming scenarios (unheated, A2, and A2 + 50%). The mesocosms had been running continuously with these treatments for 11 years prior to the HW simulation, which consisted of an additional 5 °C increase in temperature applied from 1 July to 1 August 2014. The results showed that nutrient effects on the zooplankton community composition and abundance were greater than temperature effects for the period before, during, and after the HW. Before the HW, taxon richness was higher, and functional group diversity and evenness were lower in HN than in LN. We also found a lower biomass of large Cladocera and a lower zooplankton: phytoplankton ratio, indicating higher fish predation in HN than in LN. Concerning the temperature treatment, we found some indication of higher fish predation with warming in LN, but no clear effects in HN. There was a positive nutrient and warming interaction for the biomass of total zooplankton, large and small Copepoda, and the zooplankton: phytoplankton ratio during the HW, which was attributed to recorded HW-induced fish kill. The pattern after the HW largely followed the HW response. Our results suggest a strong nutrient effect on zooplankton, while the effect of temperature treatment and the 5 °C HW was comparatively modest, and the changes likely largely reflected changes in predation.

Review of the biology of the krill genusNyctiphanesG.O. Sars, 1883 (Euphausiacea: Euphausiidae): challenges for future research on environmental change

AbstractThe genus Nyctiphanes G.O. Sars, 1883 (Euphausiacea, Euphausiidae) includes four extant species. These species are a conspicuous component of trophic webs of coastal marine ecosystems due to their abundance, the formation of dense aggregations, swarms, and schools, fast growth, and high reproductive rates. They dominate the bulk biomass in eutrophic Eastern Boundaries System and subtropical mesotrophic habitats, with estimates of 30–40% of the total zooplankton biomass. Species of Nyctiphanes are efficient omnivores and conversely prey for a large number of zooplanktonic and nektonic species. We review current knowledge of the biogeography, reproductive biology, physiology, biochemistry, ecology, and parasitology of the four species of Nyctiphanes. Most published information on Nyctiphanes focuses on the two species from the Pacific Ocean, N. australis G.O. Sars, 1883 and N. simplex Hansen, 1911, and considerably less is known on the biology and ecology of N. couchii (Bell, 1853) and N. capensis Hansen, 1911 from the Atlantic Ocean. Knowledge on the biology and ecology of the species of Nyctiphanes is still behind what is currently known for species of krill, particularly compared to Euphausia Dana, 1850 and Thysanoessa Brandt, 1851, and new multi-focal studies on the effects that environmental variables have on reproductive aspects, survival, growth, and physiology are especially critical to address under future environmental change.

Zooplankton Distribution and Community Structure in the Pacific and Atlantic Sectors of the Southern Ocean during Austral Summer 2017–18: A Pilot Study Conducted from Ukrainian Long-Liners

Preliminary results of the pilot study of the zooplankton in the region between the Ross and Scotia Seas from November 2017 to April 2018 are presented. In total, 53 zooplankton samples were collected in the top 100 m water layer using vertical tows of a 0.1 m2 Juday net from four Ukrainian longliners operating during the Antarctic toothfish fishery. Total zooplankton abundance ranged from 3 to 2836 ind m−3 with a global mean of 360 ± 550 (±1 SD) ind m−3. The highest abundances were recorded at the northeastern Ross Sea. At those stations, small copepods (mainly Oithona spp., Oncaea spp., Ctenocalanus spp. and copepod nauplii) numerically dominated the samples. Total biomass ranged from 0.3 to 85 mg DW m−3 with a mean of 10.9 ± 14.5 mg DW m−3. The highest biomasses were recorded at the eastern Ross Sea, where pelagic tunicates Salpa thompsoni, siphonophores and ctenophora Callianira sp. accounted for >90% of total zooplankton biomass. At other stations, zooplankton biomass generally ranged from 5 to 20 mg DW m−3 with no clear pattern in distribution. The community composition was driven by the sampling latitude and/or season rather than longitudinally. This pilot study emphasized the unique opportunity to investigate zooplankton dynamics in the regions traditionally not sampled during the oceanographic surveys. It also created unprecedented opportunities to increase the seasonal and geographical zooplankton sampling coverage using ships of opportunity at a fraction of a dedicated oceanographic survey costs. The potential of such surveys are enormous in both providing invaluable information, contributing to existing long-term databases and enhancing an international collaboration in the Southern Ocean, particularly in light of recent modeling initiatives of the whole Antarctic system undertaken by the Commission for the Conservation of Antarctic Marine Living Resources.

Effects of Consecutive Extreme Weather Events on a Temperate Dystrophic Lake: A Detailed Insight into Physical, Chemical and Biological Responses

Between May and July 2018, Ireland experienced an exceptional heat wave, which broke long-term temperature and drought records. These calm, stable conditions were abruptly interrupted by a second extreme weather event, Atlantic Storm Hector, in late June. Using high-frequency monitoring data, coupled with fortnightly biological sampling, we show that the storm directly affected the stratification pattern of Lough Feeagh, resulting in an intense mixing event. The lake restabilised quickly after the storm as the heatwave continued. During the storm there was a three-fold reduction in Schmidt stability, with a mixed layer deepening of 9.5 m coinciding with a two-fold reduction in chlorophyll a but a three-fold increase in total zooplankton biomass. Epilimnetic respiration increased and net ecosystem productivity decreased. The ratio of total nitrogen:total phosphorus from in-lake versus inflow rivers was decoupled, leading to a cascade effect on higher trophic levels. A step change in nitrogen:phosphorus imbalances suggested that the zooplankton community shifted from phosphorus to nitrogen nutrient constraints. Such characterisations of both lake thermal and ecological responses to extreme weather events are relatively rare but are crucial to our understanding of how lakes are changing as the impacts of global climate change accelerate.

Resilience of pond communities to extreme thermal regime shifts: an alpine–montane reciprocal transplant experiment

Elevational gradients are ideal ecological venues for testing how communities respond to environmental changes associated with global warming. Recent warming rates have been shown to increase with elevation, hypothetically translating into heat waves that more adversely affect cold-adapted alpine communities than lower montane communities exposed to cold waves. We tested this hypothesis by conducting a replicated two-factor (source × elevation) experiment involving a reciprocal transplant of regional alpine and montane pond communities across two elevational sites (2345 m versus 1390 m asl). At each elevation, half of the mesocosms were inoculated with zooplankton and sedimentary egg-banks collected from alpine ponds while the other mesocosms were similarly seeded with zooplankton and sediment collected from montane ponds in 2016. After overwintering, the mesocosms were sampled for temperature and plankton during the ice-free periods of 2017 and 2018. The mesocosms at the low elevation were 8.0 ± 1.2 °C °C warmer than those at the high elevation. Elevation significantly affected total zooplankton biomass, whereas source effects were not significant. Elevation effects revealed that the temperature difference between sites affected the phenology of species within the assembled communities, but not their total biomass. Functional trait analysis revealed that the warmer environment selected for smaller body size and asexual reproduction as a warmer thermal regime stimulated several small parthenogenetic herbivores while suppressing larger, obligate sexual omnivores. Nevertheless, thermal regime shifts did not substantially affect the abundance-weighted mean functional identities of either transferred pond community, highlighting ponds as potential sources of functional insurance against increasingly extreme climate events.

Patchy distribution of phyto- and zooplankton in large and shallow lagoon under ice cover and resulting trophic interactions

The plankton of coastal lagoons while they are covered by ice is rarely studied. We hypothesised that, under ice: (1) the lack of wind mixing would lead to strong diversification in the spatial distribution of both phyto- and zooplankton communities across the lagoon; and (2) top-down regulation would be small, and would not significantly affect the development of phytoplankton. We tested these hypotheses in Vistula Lagoon (southern Baltic Sea) a few weeks after the thickness of the ice had reached 10cm. Considerable horizontal and vertical spatial variation in some physical (turbidity, transparency) and chemical (Cl–, total N and total P) water properties were observed. According to a two-dimensional canonical correspondence analysis, these differences were related to the heterogeneous distribution of several plant and animal taxa, supporting our first hypothesis. The ratio of the total biomass of non-predatory zooplankton (BZp) to the biomass of edible phytoplankton ≤35µm (BPh) was low across the different sampling sites (mean 0.08; range 0.02–017), and differed across the lagoon. However, the significant correlation between BZp and BPh values (Spearman test, r=0.70; P≤0.05) suggests a considerable role of zooplankton filter feeders in controlling the development of phytoplankton. Therefore, top-down control may be quite strong, refuting Hypothesis 2.

Changes in the composition of the spring zooplankton of Lake Baikal

Since the 60s of the past century, researchers from Limnological Institute SB RAS have been regularly sampling zooplankton in a network of stationary stations along the Baikal water areas. In 2009-2018, the samples were collected in late May or early June immediately after the melting of ice during the complex expedition. The total biomass and abundance of zooplankton did not go beyond the interannual fluctuations observed in the 60-90s of the past century. A high abundance of spring rotifers in the Middle and Southern basins was also observed in 2015-2018. In the northern basin, the maximum rotifers indicator for the entire period of 30% of the plankton abundance was recorded only in 2016. The amount and composition of organisms in the surface layer depend on many factors. In our opinion, the depth and temperature characteristics of the water layer play an important role. The change in the heating and mixing of water could have led to observed shifts in the surface plankton composition.

ANALYSIS OF TRANSITION WATERS OF THE UKRAINIAN BLACK SEA SHELF BY ZOOPLANKTON INDICATORS (ON THE EXAMPLE OF THE DANUBE DELTA)

Problem. The Ukrainian part of the Danube Delta is a zone of constant interaction between river and sea waters, resulting in changes in their properties and formation of water masses with new qualities.According to the EU Maritime Strategy for the EU Water Framework Directive (MSFD), the new approach for determining the quality of the aquatic environment is based on the superiority of biological indicators over chemical ones. In addition, for a more detailed assessment of the quality of the aquatic environment, in accordance with the Water Framework Directive, a five-point assessment of water quality was introduced: High, Good, Medium, Low and Bad.The aim of work is to determine the ecological quality class of transitional waters by the integral index of zooplankton in the Ukrainian part of the Danube Delta.Results. To determine the water quality, zooplankton samples were taken from 2004 to 2017 at different times of the year. Water quality was determined by monthly average values, which included the data from 26 stations (spring – 51, summer – 119, autumn – 96 collected samples). The following characteristics (metrics) were selected to determine the quality of transitional waters of the Danube Delta according to indicators of zooplankton: total zooplankton biomass, Noctiluca scintillans biomass (%) of total biomass, Copepoda biomass (%) of total biomass and Shannon index by number.Conclusion. As a result of long-term monitoring (from 2004 to 2017), the obtained data of hydrological and biological indices of zooplankton in the Ukrainian part of the Danube Delta were analyzed. A long-term analysis of the quality of the Ukrainian part of the Danube Delta showed that from 2004 to 2017 the environmental quality class varied from medium to good. According to the calculated value of the integral indicator of the status of zooplankton, it was found that only summer 2007 and autumn 2016 were the most polluted periods.

Interannual dynamics of zooplankton in the Kola Section of the Barents Sea during the recent warming period

Abstract Our research focused on the analysis of interannual variability of zooplankton in the Kola Section (the Barents Sea) in the period of current warming in the Arctic basing on previously unpublished data. The zooplankton community was investigated based on the analysis of 240 plankton samples, collected in late May–early June 2009–2017. A total of 74 zooplankton taxa of nine phyla were identified in the plankton samples, but copepods were the most diverse and numerous taxonomic group. The biodiversity index varied considerably from year to year, but a stable tendency for the index to increase since the beginning of the period studied was observed. Copepods dominated in terms of abundance and biomass, comprising on average 73–96% of the total zooplankton abundance and 81–96% of the total zooplankton biomass. Calanus finmarchicus was the main zooplankton species utterly dominated by abundance and biomass (on average 92 and 97% respectively). Considerable differences in zooplankton abundance and biomass at different stations of the section were observed. Although the investigations were carried out during a warming period, interannual differences in zooplankton abundance and biomass were observed. Zooplankton biomasses were higher in the years with higher temperatures and stronger water inflow.

Winter decrease of zooplankton abundance and biomass in subalpine oligotrophic Lake Atnsjøen (SE Norway)

Despite the rapidly changing winter conditions in temperate ecosystems, little attention has been devoted to the effects of these changes on lake ecology. Few studies on the seasonal changes in abundance and biomass of the major groups of the metazooplankton community (i.e. rotifers, cladocerans and copepods) in northern oligotrophic lakes include data from the ice-covered winter months. This study reports monthly variation in zooplankton abundance and biomass from June 2010 to October 2011, including winter, in an oligotrophic, subalpine lake in southeastern Norway (Lake Atnsjøen). Changes in rotifer, cladoceran, copepod, and total zooplankton abundances and biomass were related to seasonal variation in water temperature and phytoplankton biomass by means of ordination analysis. The zooplankton abundance and biomass were much lower in winter than during the open water season. However, an under-ice phytoplankton bloom occurred during the final winter months, when snow cover and ice thickness were reduced and (presumably) light penetration increased, leading to an increase in abundance of copepod nauplii. Winter zooplankton abundance was dominated by copepods and rotifers, while winter zooplankton biomass was dominated by copepods and cladocerans. Both phytoplankton and zooplankton had two biomass peaks in 2010 and one peak in 2011. Rotifers dominated zooplankton abundance with a peak in August and total zooplankton abundance followed a similar pattern. In contrast, cladocerans dominated zooplankton biomass with a peak in July and total zooplankton biomass also peaked at this time. Rotifer and total zooplankton abundance and rotifer biomass were most closely correlated to water temperature. However, cladoceran biomass and total biomass were most closely correlated with phytoplankton biomass, but also appeared to be dependent on other carbon sources. Estimates of non-phytoplankton particulate organic carbon indicated that this part of the carbon pool could be an additional food source for zooplankton particularly in early and mid-winter. The longer growing season in 2011 than in 2010, owing to earlier ice-off in 2011, may have contributed to higher phytoplankton and zooplankton biomass in 2011. With climate warming, this is an expected change in temperate lake ecosystems.

Unusually low euphausiid biomass during the warm years of 2014–2016 in the transition zone of the California Current

Euphausiids represent a significant part of the total zooplankton biomass and are a key link in the trophic web of the California Current System (CCS). During 2014–2016, the northeast Pacific experienced warming caused by two successive events, the Blob and the 2015–2016 El Niño, which affected the whole ecosystem. Here we present evidence for a strong impact of the 2014–2016 warm events on euphausiids in the Baja California region, namely two species (Euphausia pacifica and Nematoscelis difficilis) that are dominant in the CCS transition zone. Both species decreased approximately 95% in relation to the previous years (1999–2010). A similar decrease was observed for E. pacifica during the 1997–1998 El Niño, but the decrease in N. difficilis was higher in 2014–2016 and unprecedented during the 1997–2016 period. Biomass of the coastal species Nyctiphanes simplex showed negative anomalies during the Blob but not during the 2015–2016 El Niño. The tropical species Euphausia eximia and Euphausia recurva increased in biomass during the Blob in 2014 relative to the previous years, and even more during the 2015–2016 El Niño. The strong increase of E. eximia, E. recurva, and N. simplex during El Niño may have been due to upwelling activity in winter 2015–2016. Canonical correspondence analysis (CCA) was performed to assess the relation between the euphausiid community and environmental variables. The most influential variables were temperature and chlorophyll, and to a lesser extent salinity. The subarctic E. pacifica showed covariance with chlorophyll while a set of tropical species did with thermohaline conditions. However, the variance accounted for was low (20% taking the first three axis). N. difficilis did not show significant scores in the CCA suggesting that other variables such as availability of prey could be important for this carnivore species.

Decreased influx of Calanus spp. into the south-western Norwegian Sea since 2003

The copepods Calanus hyperboreus and C. finmarchicus are the ecologically most important copepod species in the Nordic Seas. The larger C. hyperboreus, of polar origin, is advected into the south-western Norwegian Sea with the East Icelandic Current (EIC), and can therefore be considered an indicator species of these waters. Using large spatio-temporal hydrographic and zooplankton data sets, we show that an abrupt reduction in abundance of C. hyperboreus around the year 2003 in the south-western Norwegian Sea coincided with a reduction in the volume of Modified East Icelandic Water (MEIW) in the intermediate layers (R = 0.64, lagged one year). Abundance of C. hyperboreus and the volume of MEIW have since remained lower compared to the previous years (Mann-Whitney U tests, p < 0.05). The total zooplankton biomass in spring within the EIC, east of Iceland, was also much reduced around 2003 and has also since remained low. A comparable post-2003 reduction is observed in the abundance of overwintered stages of C. finmarchicus in May. Our results suggest that the reduction of zooplankton in the south-western Norwegian Sea around 2003 was caused by reduced influence of subarctic waters from the western region. This may in turn impact the extensive feeding migration of herring to this region.

Impacts of increasing nitrogen:phosphorus ratios on zooplankton community composition and whitefish (Coregonus macrophthalmus) growth in a pre‐alpine lake

Abstract The combination of increasing atmospheric depositions of reactive nitrogen (N) and the highly effective diminishing of external phosphorus (P) loadings can change key nutrient ratios in lake ecosystems. Consequently, ratios of dissolved inorganic N (DIN) to dissolved P (DP) in lakes are increasing. However, potential consequences for aquatic organisms are as yet far from understood. We formulated three hypotheses on the potential effects of rising DIN:DP ratios on a lake food web: (1) increasing DIN:DP ratios intensify the P limitation of phytoplankton communities and lower their food quality; (2) densities of P rich zooplankters (e.g. cladocerans) will be negatively affected by P‐limited food algae; (3) as result, planktivorous fish will experience a reduction of their main prey (especially Daphnia species) and respond with lowered growth. These hypotheses were tested in a mesocosm experiment conducted in a pre‐alpine lake in southern Germany, Lake Brunnensee. For 76 days, the natural phytoplankton and zooplankton communities were exposed to a wide gradient of DIN:DP ratios. At the end of the experiment, juvenile planktivorous whitefish (Coregonus macrophthalmus) were released into the mesocosms and allowed to feed on zooplankton communities for 72 hr. Along the gradient of DIN:DP ratios, we found evidence for a rising P limitation of autotroph growth, which was indicated by increasing ratios of N:P (15:1–157:1) and C:P (91:1–797:1) in seston biomass. The rising P limitation in algae reduced the nutritional food quality for the majority of herbivorous zooplankton. Both the total zooplankton biomass and the Daphnia biomass declined substantially with increasing DIN:DP ratios. In contrast, increasing DIN:DP ratios favoured rotifer species, showing strong positive correlation with rotifer biomass. Whitefish weights decreased with increasing rotifer biomass and increased with rising Daphnia biomass in zooplankton communities. In summary, our results provide an experimental demonstration that increasing DIN:DP ratios can cause stoichiometric shifts in the biomass of primary producers towards higher N:P and C:P ratios. Effects on zooplankton were changes in the taxonomical community composition towards lower cladoceran biomass (mainly Daphnia spp.). The reduction in Daphnia biomass in turn caused significantly reduced growth rates of whitefish in our experiment. Our experimental results therefore support the assumption that stoichiometric effects can travel up the food chain. General consequences of such multi‐trophic effects induced by altered nutrient ratios could be potentially visible during re‐oligotrophication of water bodies, often resulting in high N:P ratios. Further empirical studies could look for signatures of these effects on the yield of economically important species.

Short-term fish predation destroys resilience of zooplankton communities and prevents recovery of phytoplankton control by zooplankton grazing.

Planktivorous fish predation directly affects zooplankton biomass, community and size structure, and may indirectly induce a trophic cascade to phytoplankton. However, it is not clear how quickly the zooplankton community structure and the cascading effects on phytoplankton recover to the unaffected state (i.e. resilience) once short-term predation by fish stops. The resilience has implications for the ecological quality and restoration measures in aquatic ecosystems. To assess the short-term zooplankton resilience against fish predation, we conducted a mesocosm experiment consisting of 10 enclosures, 6 with fish and 4 without fish. Plankton communities from a natural lake were used to establish phytoplankton and zooplankton in the mesocosms. High biomasses (about 20 g wet mass m-3) of juvenile planktivorous fish (perch, Perca fluviatilis) were allowed to feed on zooplankton in fish enclosures for four days. Thereafter, we removed fish and observed the recovery of the zooplankton community and its cascading effect on trophic interactions in comparison with no fish enclosures for four weeks. Short-term fish predation impaired resilience in zooplankton community by modifying community composition, as large zooplankton, such as calanoids, decreased just after fish predation and did not re-appear afterwards, whereas small cladocerans and rotifers proliferated. Total zooplankton biomass increased quickly within two weeks after fish removal, and at the end even exceeded the biomass measured before fish addition. Despite high biomass, the dominance of small zooplankton released phytoplankton from grazer control in fish enclosures. Accordingly, the zooplankton community did not recover from the effect of fish predation, indicating low short-term resilience. In contrast, in no fish enclosures without predation disturbance, a high zooplankton:phytoplankton biomass ratio accompanied by low phytoplankton yield (Chlorophyll-a:Total phosphorus ratio) reflected phytoplankton control by zooplankton over the experimental period. Comprehensive views on short and long-term resilience of zooplankton communities are essential for restoration and management strategies of aquatic ecosystems to better predict responses to global warming, such as higher densities of planktivorous fish.