Chinese mitten crab <Emphasis Type="Italic">Eriocheir sinensis</Emphasis> in the Baltic Sea—a supply-side invader?

Although the Chinese mitten crab Eriocheir sinensis (H. Milne-Edwards, 1853) (Crustacea, Decapoda, Varunidae) invaded the Baltic Sea about 80 years ago, published information on its present distribution and abundance in this region is lacking. We provide here information on its Baltic-wide distribution and long-term population dynamics. The species has been found all over the coastal Baltic Sea and also in some adjacent rivers and lakes. The Chinese mitten crab appears to have increased in abundance in recent years in the northeastern part of the Baltic Sea (Gulf of Finland, Gulf of Riga, northern Baltic Proper). Higher catch rates were observed in spring (April–June) and autumn (September–November). The size variation of crabs in different samples was low (mean carapace width 6.1–6.3 cm). Despite findings of gravid females, the reproduction of the mitten crab in the central, northern and eastern Baltic region is considered unlikely due to low salinity and the individuals caught are assumed to actively migrate into the region from the species’ main European distribution area (southeastern North Sea), certainly over 1500 km migration distance. Thus, the dynamics of the North Sea population is probably regulating, at least in part, the occurrence of the Chinese mitten crab in the Baltic Sea area.     

Spatio-temporal variations of the zooplankton abundance and composition in a West African tropical coastal lagoon (Grand-Lahou,Côte d’Ivoire)

Zooplankton constitutes a sensitive tool for monitoring environmental changes in coastal lagoons; however, the available information on zooplankton communities is not sufficient to optimize their rational management. The relationships between zooplankton distribution and environmental factors were studied in a tropical lagoon to test whether the indicator properties of zooplankton assemblages could be used to monitor water quality, in a context of expected eutrophication provoked by an increasing anthropogenic activity. Twenty-one (21) stations were sampled monthly from January to December 2004. The community was composed of 65 taxa including Copepoda, Rotifera, and Cladocera. Copepoda was the most abundant group (81% of total numbers). The main zooplankton species were Oithona brevicornis, Acartia clausi, and Brachionus plicatilis. The highest zooplankton abundance (171–175 ind. l⁻¹) was recorded during the long, dry season (February–April) and the lowest (40–45 ind. l⁻¹) during the rainy and the flood periods (June–July). At a spatial level, the lowest abundance was observed in the estuarine zone. During the dry seasons (December–April and August–September), marine zooplankton taxa were abundant near the channel of Grand-Lahou, and brackish water taxa dominated in the other sites. Multivariate analyses (Co-inertia) showed that the composition of zooplanktonic communities and their spatio-temporal variations were mainly controlled by salinity variations closely linked to the climatic and hydrological context. The role of the trophic state on zooplankton communities could not be clearly evidenced. Our results and a comparison with previous studies in the neighboring, highly polluted Ebrié Lagoon suggest that the ratio between Oithona and Acartia abundance could be used as biological indicator for the water quality. Handling editor: S. I. Dodson     

Regional and seasonal variability of zooplankton collected using sediment traps in the southeastern Beaufort Sea, Canadian Arctic

Time-series sediment traps were deployed at six mooring sites in the southeastern Beaufort Sea from October 2003 to August 2004 during the cruise of the Canadian research vessel Amundsen within the framework of the Canadian Arctic Shelf Exchange Study (CASES). Trap-collected zooplankton (TCZ) at around 200 m water depth was dominated by copepods accounting for 74–93% of the total abundance throughout the year with increase in abundance at all sites during the fall. Seven distinct TCZ groups were identified through cluster analysis. Two marked seasonal shifts in TCZ composition from late fall to early winter and from spring to early summer were revealed at five sites at 200 m depth. The zooplankton was dominated by Oncaea spp., pteropods, and copepod nauplii in the late fall cluster and in the winter cluster, and by copepod nauplii in the summer cluster. A significant change in water temperature, salinity, and sea ice concentration was observed only with the spring–summer shift. The cluster analysis also revealed that TCZ composition at 200 m at a station located in the Cape Bathurst Polynya was markedly different from those at other sites through the study period by being characterized by the dominance of various copepodite stages of Metridia longa. This was probably due to a less prolonged period of sea ice cover, which provides favorable food conditions for the zooplankton community.     

舟山海域夏、秋季浮游动物的分布特征及其与环境因子的关系

为更好地了解舟山海域浮游动物的群落结构、生物量和丰度的时空分布特征及其与主要环境因子的关系,分别于2014年7月和10月进行了夏季、秋季两次生态综合调查,并用多维尺度分析法、典范对应分析法对浮游动物群落结构进行了研究。结果表明:夏季舟山海域调查的浮游动物有13类,64种,优势种为背针胸刺水蚤(Centropages dorsispinatus)、圆唇角水蚤(Labidocera rotunda)、中华哲水蚤(Calanus sinicus)、精致真刺水蚤(Euchaeta concinna)、百陶带箭虫(Zonosagitta bedoti)和真刺唇角水蚤(Labidocera euchaeta);秋季鉴定到浮游动物12类,45种,优势种为背针胸刺水蚤(Centropages dorsispinatus)、百陶带箭虫(Zonosagitta bedoti)、双生水母(Diphyes chamissonis)、瓜水母(Beroёcucumis)和中华哲水蚤。夏季浮游动物平均丰度及平均生物量(144.0 ind/m3和176.3 mg/m~3)都分别高于秋季(21.4个/m3和86.3 mg/m3);Shannon-Wiener多样性指数夏季(3.03)高于秋季(2.82),Pielou均匀度指数则是秋季(0.83)高于夏季(0.64);夏季不同区域浮游动物群落之间具有明显的差异,而秋季大部分站位群落之间差异不显著;温度、盐度、叶绿素a浓度和营养盐含量是影响舟山海域浮游动物分布的主要环境因子;与历史资料相比,舟山海域浮游动物丰度及生物量呈下降趋势,其优势种保持较稳定。     

Year-to-year changes of the mesozooplankton community in the Chukchi Sea during summers of 1991, 1992 and 2007, 2008

A recent drastic decrease in sea ice cover area was observed in the western Arctic Ocean during summer, yet little information is available for its effect on zooplankton community. To evaluate the effect of sea ice reduction on zooplankton, we studied year-to-year changes of zooplankton community structure in the Chukchi Sea during summers of 1991, 1992 (when sea ice extended), 2007, and 2008 (when sea ice reduced). Zooplankton abundance ranged from 4,000 to 316,000 ind. m⁻² (mean: 70,000) and was greater north of Lisburne Peninsula in 2008. Zooplankton biomass ranged from 0.07 to 286 g wet mass m⁻² (mean: 36) and was greater south of Lisburne Peninsula in 2007. Cluster analysis based on zooplankton abundance showed a division of the zooplankton community into four groups. Occurrence of each group was separated geographically and interannually, and geographic distributions of each group in 1991 and 1992 were similar but those in 2007 and 2008 were shifted northward. Abundance and biomass in 2007/2008 were higher than in 1991/1992, indicating that further sea ice reduction would have a positive effect on zooplankton production (e.g. invasion of large Pacific species and temperature effects on their growth rate). The northern shift in geographic distribution of the zooplankton community in 2007/2008 indicates that sea ice reduction would have a negative effect on the zooplankton community (loss of characteristic Arctic species) in part of the Chukchi Sea. These apparently contradictory effects of sea ice reduction on zooplankton community emphasize the critical need for continued monitoring in this area.     

Evaluating zooplankton indicators using signal detection theory

Indicators are used to help managers to conserve biodiversity and guarantee the sustainable use of marine resources. Good indicators are scientifically valid, ecologically relevant, respond to pressures, and it is possible to set target levels for them. Zooplankton is an important link in the food web as it transfers energy from primary producers (phytoplankton) to planktivorous fish, including the commercially important herring in the Baltic Sea. Eutrophication is known to increase zooplankton abundance and decrease the mean size of zooplankton individuals, while particularly herring prefers larger zooplankton as prey. Therefore, both the abundance/biomass and the size structure of the zooplankton community are highly relevant for the functioning of the pelagic food web, and their combination has been proposed as an indicator of the status of the pelagic food web. In this study, we evaluated the indicator performance of zooplankton abundance and mean size in the northern Baltic Sea using signal detection theory and annual zooplankton monitoring data from years 1979–2014. Herring weight-at-age and chlorophyll a levels were used to estimate the reference periods or “gold standard” of the food web status. The sensitivity and specificity of the indicator was evaluated using ROC curves. Thresholds were set and evaluated with positive and negative predictive values for the zooplankton mean size in three sub-basins of the Baltic Sea. The results suggest that zooplankton mean size is able to reflect the state of the food web in the Baltic Sea. Our study also confirms that signal detection theory is useful in evaluating ecological indicators with clear pressures. However, with parameters that are affected by multiple contrary pressures, such as zooplankton abundance, ROC curves cannot offer enough information about the performance of an indicator parameter.     

Basin scale distribution of zooplankton in the Ligurian Sea (north-western Mediterranean) in late autumn

Mesozooplankton biomass and abundance were evaluated in epipelagic waters at 59 stations covering the Italian sector of the Ligurian Sea (north-western Mediterranean) in December 1990. This region is characterised by a cyclonic circulation which encloses a central divergence zone and is associated with a main thermohaline front offshore the western Ligurian coast. At the end of autumn, mesozooplankton biomass (range: 0.80–4.24 mg DW m⁻³) and the abundance (range: 83.8–932 ind. m⁻³) were lower in the divergence zone. On the contrary, in the Ligurian frontal zone at the periphery of the divergence and on the eastern continental shelf the greatest values of biomass and abundance were recorded. Copepods and appendicularians dominated the mesozooplankton community, the main taxa being the copepods Clausocalanus spp. (46% of total zooplankton) and Oithona spp. (15%) and the appendicularian Fritillaria spp. (12%). Three hydrological sub-regions, i.e. the divergence, the eastern continental shelf and the periphery of the divergence, were characterised by different zooplankton communities and characteristic species. Environmental differences between the three zones were mainly related to changes in bottom topography, sea surface temperatures and quantity of particulate organic matter. Vertical mesozooplankton abundance and taxa distribution from the surface to 1,900 m depth were also examined in one station. The results showed that the bulk of the community was concentrated in the upper 200 m, small copepods being dominant particularly in the upper 50 m. The copepod community was more diversified in sub-superficial waters, with a maximum observed in the 200–400 m layer. The distributions of main zooplankton taxa described in epipelagic waters in the eastern Ligurian Sea in autumn were compared with their distribution at surface in the north-western Mediterranean obtained by sampling performed with the Continuous Plankton Recorder in 1997–1999. The analysis of the zooplankton community in CPR samples confirms the dominance of small copepods (Paracalanus spp., Clausocalanus spp., Oithona spp.) and appendicularians in the north-western Mediterranean in late autumn-winter and shows that their distribution is mainly related to the main mesoscale hydrographic features characterising this basin. Guest editors: S. Souissi & G. A. Boxshall Copepoda in the Mediterranean: Papers from the 9th International Conference on Copepoda, Hammamet, Tunisia     

The zooplankton consumption of primary production and an assessment of the waterbody trophic state on the basis of its structural and functional characteristics

The patterns of the seasonal dynamics of species composition, abundance, biomass, and ration of zooplankton in the Curonian Lagoon of the Baltic Sea have been considered. It is shown that zooplankton can consume up to 17–21% of the phytoplankton primary production. Of this, 7–8% of the primary production is transformed into secondary production and can be used by invertebrates and fishes. The structural and functional characteristics of zooplankton (index and coefficient of the trophic state, the number of dominating species, and the Shannon index by biomass) can be used to assess the waterbody trophic state.     

Community Structure of the Bacteria Associated with Nodularia sp. (Cyanobacteria) Aggregates in the Baltic Sea

The community structure of the bacteria associated with Nodularia spumigena (Mertens) cyanobacterial aggregates in the Baltic Sea was studied with temperature gradient gel electrophoresis (TGGE), using a 16S rRNA gene fragment as a target. Various developmental stages of the aggregates and free-floating cyanobacterial filaments were sampled to reveal possible changes in associated microbial community structure during development and senescence of the aggregates. The microbial community structures of all samples differed, and the communities of young and decaying aggregates were separated by cluster analysis of the TGGE fingerprint data. Sequencing of the TGGE fragments indicated the presence of bacteria from the α-, β-, and γ-proteobacterial groups, as well as members of Cytophaga–Flexibacter–Bacteroides lineages and gram-positive Actinobacteria spp. The majority of the Nodularia-associated sequences were not closely related to previously reported 16S rDNA sequences from the Baltic Sea or any other environment. The structure of the bacterial assemblage reflects the environmental changes associated with the succession and decay of the cyanobacterial aggregates. In addition, the sequence data suggest that the N. spumigena (Mertens) blooms in the Baltic Sea may host thus far uncharacterized bacterial species.     

Seasonal dynamics of phyto- and zooplankton and their interactions in the hypereutrophic reservoir

The seasonal dynamics of phyto- and zooplankton, as well as their trophic interactions, were studied for the shallow freshwater hypereutrophic Curonian Lagoon in the Baltic Sea in 2007–2009. The water temperature and high concentrations of nutrients were the limiting factors in the seasonal dynamics of the studied communities. Decreases in the B _z /B _ph ratio and the grazing pressure of phytophagous zooplankton were observed over the last few decades due to the eutrophication process and cyanobacteria blooms. The large zooplankton consumes a significant amount of primary production (Chlorococcales and Bacillariophyta) when the predator pressure of the fish fry is absent. This grazing pressure may precondition cyanobacteria scum.     

Copepod communities off Franz Josef Land (northern Barents Sea) in late summer of 2006 and 2007

Copepods are considered to be the main component of the Arctic marine zooplankton. We examined the copepod distribution and diversity off Franz Josef Land (northern Barents Sea) in August 2006 and 2007. A total of 18 and 14 copepod taxa were identified from the sampling layers (100–0 m or bottom–0 m) in 2006 and in 2007, respectively. There were no significant differences in the total copepod abundance between the years (means ± SE: 118,503 ± 24,115 individuals m⁻² in 2006 vs. 113,932 ± 28,564 individuals m⁻² in 2007). However, the copepod biomass in 2006 (4,518 ± 1,091 mg C m⁻²) exceeded clearly the value in 2007 (1,253 ± 217 mg C m⁻²). The copepod community showed low species richness and diversity in both years (Simpson index D: 0.34 and 0.38, respectively). Biomass of the large and small copepod species strongly decreased from 2006 to 2007. The total abundance of copepods was negatively correlated with water temperature in 2006 and positively correlated with salinity in 2007. The patchiness in copepod distribution was associated with local hydrography and temperature conditions.     

Photosynthesis and UV-B Tolerance of the Marine Alga Fucus vesiculosus at Different Sea Water Salinities

The marine algal species in the Baltic Sea are few due to the low sea water salinity. One of the few species that can be found is Fucus vesiculosus. Even this species is affected by the low salinity and becomes smaller in size in the Baltic. In present work the photosynthesis of F. vesiculosus in the northern Baltic (Bothnian Sea) was compared to the photosynthesis of F. vesiculosus in the Atlantic. Oxygen evolution was measured before and after exposure to 2.3 W of UV-B (280–320 nm) radiation for 5 h, as well as after 48 h recovery in low light. The plants were kept in their own sea water salinity as well as in a changed salinity, this to examine possible correlations between salinity and photosynthesis. The results show a significant higher initial maximal photosynthesis (P _max) for Atlantic plants (10.3 nmol O₂ g⁻¹ FW s⁻¹) compared to Baltic plants (4.0 nmol O₂ g⁻¹ FW s⁻¹). The Baltic plants were found more sensitive to UV-B with a 40–50% decrease of P _max as well as a lower degree of recovery (60–70% compared to 75–95% for the Atlantic plants). The higher salinity (35 psu) had a positive effect on the Baltic F. vesiculosus with increased P _max as well as increased tolerance to UV-B. The lower salinity (5 psu) had a negative effect on the Atlantic plants with a decreased P _max as well as a lower tolerance to UV-B. Pigment content was found higher in Atlantic F. vesiculosus. The pigment content decreased then the Atlantic plants were transferred to 5 psu. The concentration of Chl a as well as the total content of violaxanthin, diadinoxanthin and zeaxanthin in Baltic plants increased when transferred to 35 psu. The Atlantic F. vesiculosus can not survive the low salinity in the northern Baltic (died within 8 weeks). It is likely that a long time acclimation or adaptation to low salinity has taken place for F. vesiculosus in northern Baltic. If this is an ecotypic or genotypic development it is too early to say.     

Distribution Patterns of Isomorphic Cold-Water Dinoflagellates (Scrippsiella/Woloszynskia Complex) Causing ‘red tides’ in the Baltic Sea

During the latest years medium-sized (15–30 μm), single-celled dinoflagellates have been reported to form blooms in the northern Baltic Proper and the Gulf of Finland in winter and spring. Recent studies (Kremp et al., 2003. Proceedings of the 7th International conference of Modern and Fossil Dinoflagellates, September 21–25, Nagasaki, Japan, 66 pp.) indicate that those blooms are caused by two isomorphic species – Scrippsiella hangoei (Schiller) Larsen, and a new species, tentatively belonging to the genus Woloszynskia. Until now there has been no report on how widely distributed these phytoplankton species are in the Baltic Sea. In this study, the occurrence of Scrippsiella/Woloszynskia complex in the entire Baltic Sea was investigated, by using monitoring data from 1997 to 2003. The species occurred in a salinity range from 2 to 8 PSU. Highest concentrations were observed at salinity 4.5–6.5 PSU. Maximum cell densities of Scrippsiella/Woloszynskia complex in the water column were mainly obtained in April or in the beginning of May by the water temperature <3 °C prior to stratification was formed. In the central Gulf of Finland, the second maximum was found in 1999 and 2002 by the temperature >6 °C. Bloom formations in the Baltic Proper and in the Gulf of Finland may not only be explained by optimum temperature and salinity, but also with other factors e.g. high nutrient concentrations and good seeding conditions from the sediments.     

Changes in biodiversity of phytoplankton,zooplankton, fishes and macrobenthos in the Southern Caspian Sea after the invasion of the ctenophore <Emphasis Type="Italic">Mnemiopsis Leidyi</Emphasis>

Monitoring for 6 years (2001–2006) showed that the population explosion of the alien ctenophore Mnemiopsis leidyi in the southern Caspian Sea coincided with a decline in the abundance and species number of mesozooplankton. While this decline appeared to have reduced the nourishment of sprat (also known as kilka), it seemed to have affected phytoplankton favorably mainly due to the decrease in grazing pressure. During 2001–2002, when M. leidyi abundance and biomass were at their highest levels, abundance of dinoflagellates and cyanophytes exceeded that of diatoms. Before the invasion (1996) and in some years after the invasion (2003, 2004 and 2006) diatom abundance was higher than the abundance of other groups. In September 2005, an unprecedented bloom of the toxic cyanophyte Nodularia sp. was observed in the southern Caspian Sea. Disappearance of edible zooplankton such as Eurytemora spp. was among the first changes observed after the expansion of M. leidyi in the area. Some changes in the macrobenthic fauna were also conspicuous after the increase of this ctenophore. While the biomass of some deposit feeders, such as the polychaete Nereis diversicolor and oligochaete species increased, benthic crustaceans decreased sharply in abundance during 2001–2003 and completely disappeared during 2004–2006. Iranian catches of kilka, the most abundant and widespread zooplanktivorous fish, decreased significantly in the southern Caspian Sea after 1999. Iranian landings of kilka dropped ~70% from 69,070 ± 20,270 t during 1995–2000 to 23,430 ± 12,240 t during 2001–2006, resulting in a loss of at least 125 million US dollars to the economy. There were also changes in the total catches of large predators such as the kutum and mullet, which mainly feed on kilka, between 1991 and 2006.     

中国近海浮游动物群落结构及季节变化

2006年7-8月、12月-2007年2月、2007年4-5月、2007年10-12月,对中国近海进行了4个航次生物、化学和水文等专业综合调查.根据采集的浮游动物样品分析鉴定结果,对中国近海浮游动物群落结构、种类组成及优势种的季节变化进行研究.结果表明,中国近海浮游动物有1330种,隶属于7门19大类群,浮游幼体47类,其中,节肢动物为最优势类群,有782种,占58.80％,其次为刺胞动物,有324种,占24.36％.在浮游动物群落结构中,4个海区均以桡足类和水母类的种类和数量占绝对优势.中国近海4个海区浮游动物种类数有明显季节变化,渤海和黄海,浮游动物种类数夏、秋季多于春、冬季;东海和南海,浮游动物种类数春、夏季多于秋、冬季.中国近海浮游动物群落大体可划分为6个主要生态类群:近岸低盐类群、低温高盐类群、高温高盐类群、低温广盐类群、高温广盐类群和广温广盐类群.结合同步调查的其它生物、水文、化学环境参数的分析结果,对中国近海浮游动物群落种类丰度与环境因子进行生物与环境变量关系分析,结果表明,浮游动物群落结构与水温、盐度、水深、溶解氧、硝酸盐和pH存在明显相关关系.水温和盐度是影响浮游动物群落结构最重要的两个环境因子.     

Ecology and long-term forecasting of sprat (<Emphasis Type="Italic">Sprattus sprattus balticus</Emphasis>) stock in the Baltic Sea: a review

In the brackish Baltic Sea situated in the transition zone between the Atlantic and Euro-Asiatic continental climate systems, the periods of high abundance of sprat of marine boreal origin coincide with a rich freshwater discharge, large water volume inhabitable by fish, rather high winter temperature and low salinity limiting the stock of its main predator—cod (Gadus morhua callarias). In the freshening periods an additional volume of water with acceptable oxygen and temperature conditions for sprat is formed in the Eastern (E) and, especially, the Northwest (NW) regions of the Baltic Sea. This allows extraordinary increase in sprat abundance/biomass. The conditions for sprat deteriorate during the period of active saline water inflows and colder winters, especially in the NW region. Following the decrease in the volume of water acceptable for the wintering of sprat as to temperature and/or oxygen concentration, some part of the stock may be forced to migrate southwards as in the Southwest (SW) region the conditions are the most stable. This may cause an extensive mixing of the stock components of various regions, hindering the differentiation of sprat regional units in the Baltic Sea. As no isolation of regional spawning concentrations has been found, beginning with 1989 the Baltic sprat has been assessed and managed as one stock unit, despite spatial differences in its morphological characters, growth rate etc. The alternation of periods of different regime-forming conditions (freshening or oceanization) is probably triggered by climate changes. This regularity has been exploited for the composition of long-term forecasts of qualitative changes in fish (incl. sprat) stocks in the Baltic Sea.     

Inter‐annual variation in herring,Clupea harengus,and sprat,Sprattus sprattus,condition in the central Baltic Sea: what gives the tune?

The Baltic Sea ecosystem has undergone large changes during the last two decades, including a severe reduction in cod and herring biomass but, at the same time, a large increase in sprat abundance. The lower trophic levels of the Baltic Sea also changed due to environmental fluctuations, including variations in salinity and in volume of oxygenated water. In this apparently shifting environment, the conditions of herring and sprat have undergone large inter-annual variations during the past 15–20 years. In this study, we explore how abiotic factors (i.e. salinity and temperature) and biotic factors (biomass of the copepods Pseudocalanus elongatus , Temora longicornis , Acartia spp. and of cladocerans as well as clupeid abundance) in different seasons (May and August) affect clupeid body condition. Our analyses suggest that data of zooplankton biomass and abiotic factors in August have higher predictive power than May data. Although our analysis suggests that salinity (a bottom-up process) has an effect on sprat condition, total abundance of clupeids (a top-down process) is by far the most significant predictor of both herring and sprat condition. The strong correlation between clupeid abundance and total zooplankton biomass points to food competition and to top-down control by herring and sprat on common food resources. Furthermore, clupeid condition co-varied with the changes in the weight of zooplankton in the stomachs, which further suggest food competition being the main mechanism behind the changes in clupeid condition during the last two decades. Hence, our results are not in agreement with most of the current literature that has suggested that clupeid growth is regulated by environmentally mediated bottom-up processes acting on the abundance of copepods. This is, to our knowledge, the first evidence of food resources mediated density-dependent fish growth in a large marine ecosystem.     

Evidence for active microbial nitrogen transformations in sea ice (Gulf of Bothnia, Baltic Sea) in midwinter

Nutrient concentrations, chlorophyll-a, bacterial biomass and relative activity of denitrifying organisms were investigated from ice-core, brine and underlying water samples in February 1998 in the Gulf of Bothnia, Baltic Sea. Examined sea ice was typical for the Baltic Sea; ice bulk salinity varied from 0.1 to 1.6 psu, and in underlying water salinity was from 4.2 to 4.7 psu. In 2- to 3-months-old sea ice (thickness 0.4–0.6 m), sea-ice communities were at the winter stage; chl-a concentrations were generally below 1 mg m⁻³ and heterotrophic organisms composed 7–20% of organism assemblage. In 1-month-old ice (thickness 0.2–0.25 m), an ice spring bloom was already developing and chl-a concentrations were up to 5.6 mg m⁻³. In relation to low salinity, high concentrations of NH⁺ ₄, NO⁻ ₂, PO³⁺ ₄ and SiOH₄ were found in the ice column. The results suggest that the upper part of ice accumulates atmospheric nutrient load during the ice season, and nutrients in the upper 10–20 cm of ice are mainly of atmospheric origin. The most important biological processes controlling the sea-ice nutrient status are nutrient regeneration, nutrient uptake and nitrogen transformations. Nutrient regeneration is specially active in the middle parts of the 50- to 60-cm-thick ice and subsequent accumulation of nutrients probably enhances the ice spring bloom. Nitrite accumulation and denitrifying activity were located in the same ice layers with nutrient regeneration, which together with the observed significant correlation between the concentrations of nitrogenous nutrients points to active nitrogen transformations occurring in the interior layers of sea ice in the Baltic Sea. Accepted: 12 June 2000     

Zooplankton response to a warmer northern Wadden Sea

Weekly measurements of mesozooplankton (>76 μm) and hydrographic parameters have been carried out since 1984 in the List Tidal Basin (northern Wadden Sea). Monthly water temperature significantly increased by 0.04°C year⁻¹. The largest increase by 3°C in 22 years occurred in September, implying an extension of the warm summer period. Mean annual copepod abundance and length of copepod season correlated significantly with mean temperature from January to May. Except for an increasing Acartia sp. abundance during spring (April–May), no long-term trends in copepod abundance were observed. The percentage of carnivorous zooplankton increased significantly since 1984 mainly due to a sudden increase in the cyclopoid copepod Oithona similis in 1997. We expect that global warming will lead to a longer copepod season and higher copepod abundances in the northern Wadden Sea.