Abundance Of Chaetognaths Research Articles

Interannual changes in the zooplankton community structure on the southeastern Bering Sea shelf during summers of 1994–2009

On the southeastern Bering Sea shelf, mesozooplankton plays an important role in material transfer between primary producers and fisheries resources. The biomass of mesozooplankton in this region is known to vary annually, but little is known about annual changes in community structure and species composition. In the present study, regional and long-term changes in abundance, biomass and community structure of copepods and chaetognaths on the shelf were evaluated based on NORPAC net samples collected during summers of 1994–2009. During the study period, regime shifts occurred from high interannual variability regime (1994–1999) to low interannual variability regime with high temperature (2000–2005), then to a low interannual variability regime with low temperature (2007–2009). A total of 24 calanoid copepod species belonging to 21 genera were identified from samples. Copepod abundance ranged from 150 to 834,486inds.m−2, was greatest on the Middle shelf, and was higher in cold years, than in warm years. Copepod biomass ranged from 0.013 to 150gDMm-2, and was also higher in cold years than in warm years. Based on the results of cluster analysis, the copepod community was divided into six groups (A–F). The regional and interannual distributions of each group were distinct. Interannual changes in abundance of the dominant copepod on the Outer shelf and Middle shelf were highly significant (p<0.0001), and their abundances were negatively correlated with temperature and salinity. Interannual changes in copepod community that occurred between cold and warm years are thought to have been caused by differences in the magnitude and timing of the spring phytoplankton bloom between the two regimes. Abundance and biomass of the chaetognath Parasagitta elegans ranged from 30 to 15,180inds.m-2 and from 11 to 1559mgDMm−2, respectively. Chaetognath abundance was significantly correlated with the abundance of the dominant copepods (p<0.0001). Differences in cold and warm years may also affect recruitment of walleye pollock. We conclude that on the southeastern Bering Sea shelf, the magnitude and timing of primary production, which is related to climate change, may significantly affect how it is transferred through the food web.

Zonal variations in abundance and body length of chaetognaths in the 140°E seasonal ice zone during the austral summer of 2001/02

Time-series observations of chaetognaths were carried out during four cruises along the 140°E transect between 61°S and 66°28′S from November to March in the 2001/02 austral summer. Three species –Eukrohnia hamata, Sagitta gazellae and Sagitta marri – occurred in the samples between 0 and 150 m. E. hamata was the most dominant species comprising between 89.6 and 100% of the chaetognath population, followed by S. gazellae (0–5.7%). There were large differences in the abundance and size frequency distribution of body length of E. hamata between the north and south of the Southern Boundary of the Antarctic Circumpolar Current (SB-ACC) which was located between 64°S and 65°S. For E. hamata, low abundance and large sized animals (22–24 mm) occurred south of the SB-ACC. A possible reason could be that the breeding season in waters north of the SB-ACC may be early spring and summer. On the other hand, low reproduction was recognized by low the abundance of E. hamata and few occurrences of juveniles south of the SB-ACC (65°S). The result of a general comparison suggests that the abundance of chaetognaths along the 140°E transect has decreased during the 20 years since 1983.

Mesozooplankton and copepod community structures in the southern East China Sea: the status during the monsoonal transition period in September

A field sampling was conducted before the onset of the northeasterly monsoon to investigate the copepod community composition during the monsoon transition period at the northern coast of Taiwan (East China Sea). In total, 22 major mesozooplankton taxa were found, with the Calanoida (relative abundance: 66.36%) and Chaetognatha (9.44%) being the most abundant. Mesozooplankton densities ranged between 226.91 and 2162.84 individuals m−3 (mean ± SD: 744.01 ± 631.5 individuals m−3). A total of 49 copepod species were identified, belonging to 4 orders, 19 families, and 30 genera. The most abundant species were: Temora turbinata (23.50%), Undinula vulgaris (17.92%), and Acrocalanus gibber (14.73%). The chaetognath Flaccisagitta enflata occurred at all 8 sampling stations, providing a 95% portion of the overall chaetognath contribution. Amphipoda were abundant at stations 4 and 5, with Hyperioides sibaginis and Lestigonus bengalensis being dominant, and comprising about 50% of all amphipods. Chaetognath abundance showed a significantly negative correlation with salinity (r = 0.77, p = 0.027), whereas mesozooplankton group numbers had a significantly positive correlation with salinity (r = 0.71, p = 0.048). Densities of four copepod species (Calanus sinicus, Calocalanus pavo, Calanopia elliptica and Labidocera acuta) showed a significantly negative correlation with seawater temperature. Communities of mesozooplankton and copepods of northern Taiwan varied spatially with the distance to land. The results of this study provide evidence for the presence of C. sinicus in the coastal area of northern Taiwan during the early northeast monsoon transition period in September.

Distribution and feeding of chaetognaths in the epipelagic zone of the Lazarev Sea (Antarctica) during austral summer

This study seeks to determine the effects of local hydrography on the distribution, abundance and feeding of chaetognaths in the Lazarev Sea, an area strongly controlled by physical processes which has been held responsible for initiating the Weddell Polynya. Zooplankton samples were taken at 39 stations on four transects located between 6°W and 3°E and from 60°S to 70°S between surface and 350 m. The dominant species, Eukrohnia hamata, accounted for 86.5% of all chaetognaths, followed by Sagitta gazellae (8.1%) and Sagitta marri (5.4%). These three species showed distinct vertical and horizontal distribution patterns. While E. hamata and S. marri had maximum abundances below 250 m depth, S. gazellae showed a narrow distribution band in the upper 150 m depth. The distribution pattern was strongly modified at the Greenwich meridian with an upward transport of a high abundance of deep dwelling organisms (S. marri and E. hamata) and a displacement of S. gazellae to the surface, likely coupled with the rise of the warm, saline halo around the Maud Rise. Small copepods were the main prey of all three chaetognath species. Feeding rates (FR) varied among species and depth. Sagitta marri showed the highest FR with 0.38 prey d−1, followed by S. gazellae and E. hamata (0.22 and 0.07 prey d−1). Feeding rates were usually highest in the 25–80-m stratum. Size distribution and maturity of E. hamata revealed a dominance of small and immature organisms along all depths and stations, suggesting that this area might be acting as an important source of recently spawned organisms to the surface.

Impact of oxygen-depleted water on the vertical distribution of chaetognaths in the northeastern Arabian Sea

The influence of a thick layer of oxygen-depleted water (<0.2 ml l −1) on the abundance and distribution of chaetognaths was investigated in the northeastern Arabian Sea (NEAS), a natural oxygen-deficient system in the global ocean. The species and maturity stage-wise distribution of this group were studied at five discrete depths down to 1000 m. A total of 22 species belonging to four genera were observed, and the genus Sagitta dominated, representing 60% (500–1000 m) to 89% (Mixed layer depth) of the total chaetognath population. Based on their vertical distribution limits, four groups were recognised, as follows: I: species abundant in surface water with a maximum distribution limit up to 300 m; II: species confined mainly to deeper waters (>500 m); III: species present throughout the water column (0–1000 m); and IV: species present in most layers, but with a preference for a specific depth stratum. A positive correlation ( P<0.01) was observed in the abundance of chaetognaths and their main prey copepods, emphasising the strong trophic relationship between these groups. It was found that the intensely oxygen-deficient waters of the NEAS play a crucial role in the vertical distribution and abundance of chaetognath species of all four genera. This report presents information on the maturity stages and ontogenetic migration of this important planktonic group in relation to the oxygen-depleted water in the study region for the first time. The results obtained are also important for understanding the biological processes associated with a major oxygen minimum zone (OMZ) in the global ocean.

Role of midwater chaetognaths in Southern Ocean pelagic energy flow

We estimate the energy flow through mesoand bathypelagic chaetognaths in the Atlantic sector of the Southern Ocean from (1) depth-structured chaetognath abundance and body mass data, (2) a general chaetognath respiration model driven by body mass, temperature, water depth and taxon, and (3) published relationships between respiration, production and consumption in chaetognaths. In the 500 to 2000 m depth layer chaetognaths have a mean biomass of 0.109 mg C m–3 in summer and 0.146 mg C m–3 in winter. Chaetognaths respiration and consumption amount to 282 and 563 mg C m–2 yr–1, respectively. Thus, Antarctic midwater chaetognaths consume 0.05% of the copepod standing stock per day or 1% of the daily copepod production in summer. About 2.8% (= 1.9 g C m–2 yr–1) of the net annual primary production is required to fuel the midwater chaetognath community via herbivorous copepods. When assuming a 1:1 diet of herbivorous and carnivorous copepods, this share increases to 6.1% (= 4.1 g C m–2 yr–1) of annual primary production. It is estimated that chaetognath consumption for the whole water column is 1.4 g C m–2 yr–1. This corresponds to 7.1% (= 4.8 g C m–2 yr–1) and 15.5% (= 10.4 g C m–2 yr–1) of the primary production channeled through herbivorous copepods and through herbivorous and carnivorous copepods, respectively. Hence, chaetognaths represent an important link between lower and higher trophic levels. To further unravel their role in the ecosystem, additional studies on the mesoand bathypelagic zooplankton community are needed.

Horizontal and vertical distribution of Chaetognatha in the upper 1000 m of the western Sargasso Sea and the Central and South-east Atlantic

The chaetognath abundance, species richness and bathymetric distribution in the upper 1000 m in two regions of the Atlantic Ocean is discussed based on samples collected on two cruises, one to the Sargasso Sea (Northwest Atlantic) on board the R/V R.H. Brown in April 2006, the other along the eastern boundary of the Atlantic (Canary Islands to South Africa) on board the FS Polarstern in November 2007. During both cruises the MOCNESS-1 was used for the sampling. Seventeen species were found in the Sargasso Sea (Ron Brown Cruise) and twenty along the eastern boundary of the Atlantic (the Polarstern Cruise). Sagitta hispida and Krohnitta mutabbii were restricted to the Sargasso Sea, and Sagitta tasmanica, S. gazellae, Eukrohnia macroneura and Krohnitta pacifica to the East Atlantic. In the East Atlantic the most abundant species were Pterosagitta draco, Sagitta enflata, S. serratodentata and Krohnitta subtilis, and in the Sargasso Sea, the most abundant species in Stations 1 and 2 was Sagitta minima. In the other stations S. serratodentata, S. enflata, Pterosagitta draco and Krohnitta subtilis were most abundant. Sagitta zetesios, S. planctonis, S. decipiens, S. sibogae and Eukrohnia fowleri were present at all stations but always in low numbers.

Distribution of chaetognaths in the central part of the Drake Passage

The mesoscale distribution of two chaetognath species, Eukrohnia hamata and Sagitta gazellae, found in the Drake Passage in October and November of 2008 was studied using a stationary grid in the area of the Antarctic Polar Front. The distance between the stations was around ten miles. The most abundant chaetognath species was Eukrohnia hamata, which accounted for 86.7% of all the individuals collected. In the upper 200 m, the average abundance of chaetognaths decreased from 2.1 to 0.8 ind./m3, and their average biomass decreased from 1.25 to 0.25 mg/m3 over the transition through the front from the north to the south. The high heterogeneity of the spatial chaetognath distribution in the area was demonstrated: the abundance of the chaetognaths could increase or decrease over 20 miles by a factor of 45 (6.8 and 0.15 ind./m3). In addition, two peaks were revealed in the size structure of the E. hamata population, probably indicating two periods of reproduction in this species in the Drake Passage.

Abundance of pelagic chaetognaths in northwestern Persian Gulf

The abundances and species compositions of chaetognaths were investigated on the northwestern coasts of the Persian Gulf in Khuzestan province (2004-2005). Highest mean chaetognath abundance was observed during the summer (324.7� 97.6 indiv. m � 3 ), while lowest values occurred during the winter (79.3� 18.6 indiv. m � 3 ). Six species were identified, Sagitta bedfordi, S. enflata, S. ferox, S. hexaptera, S. neglecta and Krohnitta pacifica. Sagitta ferox was most abundant. It represented 49.7% of total chaetognath numbers. Abundance variation throughout the year cycle seemed to be related to environmental factors. The correlation coefficients between abundance of chaetognaths and the water temperature were significantly positive but there was no significant relation for abundance and salinity. However, salinity values in the Persian Gulf are very high compared with other regions and perhaps pelagic chaetognaths species that inhabit this region are adapted to very high salinities of up to 45.

Chaetognaths of the United States South Atlantic Bight: Distribution, abundance and potential interactions with newly spawned larval fish

Chaetognaths are one of the most numerous organisms in the zooplankton community off the coast of North Carolina. During two years of offshore sampling in the late winter to early spring, sixteen chaetognath species were identified, four of which had not previously been reported in the waters of the United States South Atlantic Bight. Offshore samples were dominated by Sagitta enflata Grassi, 1881, one of the larger species, which contributed > 61% of total chaetognath abundance while dominant coastal species were S. tenuis Conant, 1896 and S. hispida Conant, 1895. Abundances, body sizes and spatial distributions were determined for the most abundant chaetognath species along with the overall abundance of three common co-occurring larval fish species (spot, Atlantic croaker and Atlantic menhaden). In addition, laboratory feeding experiments were conducted using S. tenuis and S. hispida to estimate the potential impact of chaetognath predation on representative North Carolina larval fish which spawn offshore and subsequently migrate into local estuaries. Feeding rates (no. prey items day − 1 ) varied with prey type and the condition (starved/fed) of the chaetognath. Weight specific daily rations (SDR) were found to vary inversely with chaetognath size, decreasing exponentially with an increase in chaetognath length. The observed abundance and distribution data indicate that wintertime chaetognath populations in offshore waters of Onslow Bay, North Carolina have the potential to interact with recently spawned larval fish and may simultaneously act as competitors, predators, or prey. Furthermore, calculations using published values of chaetognath abundance, zooplankton standing crops, and our SDR estimates indicate that chaetognaths in a representative North Carolina estuary would require a minimum of 5.96 cal m − 3 day − 1 to sustain their biomass. Allowing for the reported spatial and temporal variability in zooplankton abundance in these systems, chaetognaths should consume 4.4%–20.9% of the estimated total zooplankton production day − 1 . This further emphasizes the role of chaetognaths not only as predators, but also as competitors with larval fish for zooplankton food stocks in southeastern United Sates estuaries.

Meso- and bathypelagic distribution and abundance of chaetognaths in the Atlantic sector of the Southern Ocean

We conducted multinet sampling during winter and summer in the Southern Ocean (Atlantic sector) to investigate the effect of water mass, season and water depth on abundance and species composition of meso- and bathypelagic chaetognaths. Eukrohnia hamata (mean 115 ind. 1,000 m−3) and Sagitta marri (mean 51 ind. 1,000 m−3) were dominant, complemented by E. bathypelagica (mean 19 ind. 1,000 m−3) and E. bathyantarctica (mean 19 ind. 1,000 m−3) below 1,000 m. A further six species were identified, among them the rare bathypelagic species Heterokrohnia fragilis and the subtropical Eukrohnia macroneura that is new to the Antarctic. Water depth and season were the principal determinants of abundance and species composition patterns, indicating vertical seasonal migration and vertical segregation of species. The life cycles of E. hamata and S. marri were studied additionally. Their maturity stages were vertically segregated and prolonged reproductive periods are suggested for both species.

Dynamics of structural and functional characteristics of zooplankton in the Northern Sea of Okhotsk in the “Warm” and “Cold” years

An analysis of the structural and functional characteristics of zooplankton community and their coupling with the synoptic situation was carried out for the spring period in the northern Sea of Okhotsk. The years of 2000 and 2001 may be referred to as “cold” type due to the significant extent of the ice-covered area. Based on the ice cover extent, the years of 2004 and 2005 may be referred to as “warm” years due to the decreased extent of the ice-covered area. The “cold” and “warm” years did not significantly differ from each other in terms of the total zooplankton biomass. In the northeast Sea of Okhotsk, the zooplankton abundance increased during the “cold” years, while, in its northwestern part, the zooplankton abundance increased during the “warm” years. The different types of years significantly differ in the species structure of the macrozooplankton community. The abundance of copepods was higher in the “warm” years, whereas the chaetognath abundance increased in the “cold” ones. These features of the dynamics and distribution of zooplankton in the northern part of the Sea of Okhotsk also affected its productivity. The higher production of the elements of the zooplankton community that was revealed in the “warm” years was caused by the increase in the seasonal P/B ratio of both nonpredatory and predatory species. In the “warm” years, the production of nonpredatory zooplankton was higher in the northwest, while, in the “cold” years, it was higher in the northeast of the area.

Spatial distribution of chaetognaths off the northern Bicol Shelf, Philippines (Pacific coast)

Abstract Noblezada, M. M. P., and Campos, W. L. 2008. Spatial distribution of chaetognaths off the northern Bicol Shelf, Philippines (Pacific coast). – ICES Journal of Marine Science, 65: 484–494. The composition, abundance, and distribution of chaetognaths off the northern Bicol Shelf, Philippines (Pacific coast), from 31 stations along transects perpendicular to the coast were analysed. Samples were collected in April, 2001. In all, 26 species belonging to 14 genera were identified. Flaccisagitta enflata was the most abundant and frequently captured species at all stations, and constituted 41.9% of the total specimens. Most of the smallest diversity values were observed from areas affected by upwelling, although the greatest densities were observed at stations located within the upwelling zones. The occurrence of mesopelagic and bathypelagic species (Decipisagitta decipiens, Caecosagitta macrocephala, and Eukrohnia fowleri), in samples collected from upper water layers, could be explained by vertical transport caused by upwelling.

Abundance, vertical distribution and feeding of chaetognaths in the upper 50 m layer of the eastern Aegean Sea

The present study presents data on the abundance, vertical distribution and feeding of chaetognaths found in the 0–50 m column in a neritic area of the eastern Aegean Sea during July 2004. The abundance of total chaetognaths ranged between 7.76 and 35.92 ind m−3. The highest values were recorded in the uppermost 20 m and values decreased with depth. The chaetognath community comprised nine species with Sagitta enflata, Sagitta minima, Sagitta serratodentata and Sagitta setosa being numerically the most important. Copepods were the main prey for most chaetognath species, while S. minima showed a higher preference for cladocerans. An evident relation between the presence of food of chaetognaths and their horizontal and vertical distribution in the area of study was found. In the surface 0–50 m of this typical oligotrophic area of the eastern Mediterranean, the chaetognath species occupied different depth strata possibly in order to reduce the interspecific competition.

Chaetognath species-specific responses to climate regime shifts in the Tsushima Warm Current of the Japan Sea

Expanding the work in our previous paper (Nagai et al. 2006), which showed the close linkage of chaetog- nath species to changes in water temperature and reported their occurrence characteristics, chaetognath species-specific responses to climate forcing via water temperature fluctuations were ascertained by studying inter-annual linkages among climate-ocean-ecosystem elements. Using samples collected between 1972 and 2002 in the Japan Sea, an attempt was made to understand how the three dominant chaetognaths—Sagitta minima, S. nagae and S. enflata—re- spond to inter-annual variations of atmospheric signals and oceanic effects in the Tsushima Warm Current around cli- mate regime shift years. Among ocean-atmosphere parameters, the winter monsoon was the most crucial for driving the ecosystem of the Japan Sea. No significant correlation with any of the other parameters was found with the southern os- cillation index, but the winter monsoon index had a correlation with water temperature. The climate regime shift during 1976/77 occurred with anomalously cold water induced by a strong winter monsoon. The next climate regime shift (1988/89) appeared with a warm water anomaly due to retention of warm water formed in winter by a weak winter monsoon and increased transport volume in the Tsushima Warm Current water. Chaetognath abundance and species numbers decreased during the colder regime and increased during the warmer regime. Responding to temperature in a species-specific manner, chaetognaths varied notably in abundance. Consequently, changes in species numbers and switching of dominant species occurred. Thus, chaetognath species could be a significant indicator of climate events that influence the ecosystem in the Japan Sea.

Species composition and seasonal abundance of Chaetognatha in the subtropical coastal waters of Hong Kong

Species composition, species diversity and seasonal abundance of chaetognaths were studied in Tolo Harbour and the coastal waters of eastern Hong Kong. Tolo Harbour is a semi-enclosed and poorly flushed bay with a long history of eutrophication. It opens into the eastern coast of Hong Kong which is fully exposed to water currents from the South China Sea. Zooplankton samples were collected monthly from July 2003 to July 2005 at six stations. Twenty species of chaetognaths were identified. They included six species of the genus Aidanosagitta ( Aidanosagitta neglecta, Aidanosagitta delicata, Aidanosagitta johorensis, Aidanosagitta regularis, Aidanosagitta bedfordii and Aidanosagitta crassa), four species of the genus Zonosagitta ( Zonosagitta nagae, Zonosagitta bedoti, Zonosagitta bruuni and Zonosagitta pulchra), three species of the genus Ferosagitta ( Ferosagitta ferox, Ferosagitta tokiokai and Ferosagitta robusta) and one species each from the genera Serratosagitta ( Serratosagitta pacifica), Decipisagitta ( Decipisagitta decipiens), Flaccisagitta ( Flaccisagitta enflata), Krohnitta ( Krohnitta pacifica), Mesosagitta ( Mesosagitta minima), Pterosagitta ( Pterosagitta draco) and Sagitta ( Sagitta bipunctata). The most abundant species were Flaccisagitta enflata, A. neglecta and A. delicata. Averaged over the entire study period, the densities of Flaccisagitta enflata, A. neglecta and A. delicata were 9.3, 6.6 and 5.2 ind. m −3, respectively. Overall, these species constituted 39.7%, 28.2% and 22.0% of all chaetognaths collected in the study. Averaged over the entire study, the density of most of the low abundance species was <0.6 ind. m −3. Flaccisagitta enflata occurred throughout the year at all sampling stations. Aidanosagitta neglecta occurred at all sampling stations, but was most common in summer. Aidanosagitta delicata was most common in Tolo Harbour during summer. Tolo Harbour supported larger populations, but fewer species of chaetognaths than the surrounding open waters. These observations suggest that some species of chaetognaths were carried into the coastal waters of Hong Kong by water currents from the South China Sea and established large populations in the productive and poorly flushed waters of Tolo Harbour.

Abundance and community structure of chaetognaths from the epipelagic through abyssopelagic zones in the western North Pacific and its adjacent seas

Abundance and community structure of chaetognaths were studied based on the vertical stratified zoo-plankton samples from the epipelagic through abyssopelagic zones (maximum: 5,000-5,800 m) at four stations in the western North Pacific (44°N, 39°N, 30°N, and 25°N) and one station each in the Japan Sea, Okhotsk Sea, and Bering Sea. Chaetognath standing stocks (indiv. m-2) were greatest at subarctic stations (44°N, Okhotsk Sea, and Bering Sea). Vertically, chaetognath density (indiv. m-3) was the greatest in the shallowest depths, and decreased with increasing depth. Chaetognaths occurred down to 4,000-5,000 m at subarctic stations, while chaetognaths were not found below 3,000 m at subtropical stations (30°N and 25°N). The number of chaetognath species was the greatest (22 representing 14 genera) at the transition station (39°N), and the least (1 species) at the station in the Japan Sea. Species diversity indices (H ') were low at subarctic stations, but high at subtropical stations. Vertical profiles of H ' varied also between these stations; it peaked at the mesopelagic zone at subarctic stations, and at the epipelagic zone at subtropical stations. Cluster analysis separated chaetognath communities of the study region into 5 groups (A-E) characterized by discrete spatial distribution: group A; the mesopelagic zone at subtropical and transition areas, group B; the epipelagic zone at subtropical and transition areas, group C; the bathy- and abyssopelagic zone (except the Japan Sea), group D; all depths in the Japan Sea, and group E; the epi- and mesopelagic zones in the subarctic area. For the four most abundant species at the subarctic stations, allometric data showed greater head width to total length ratios, and greater hook length to total length ratios for deeper-dwelling species. Relatively larger head width (i.e. large mouth) and longer hooks of deep-sea chaetognaths are considered to be an adaptation significant to the successful capture of prey zooplankton in resource limited deep-sea environments.

Quetognatos: ¿Indicadores de eventos climáticos anómalos en el Océano Pacífico Colombiano?

This principal aim of this work was to establish the value of chaetognaths as indicators of El Nino and La Nina events in the Colombian Pacific Ocean. The studied periods were 23.Jun-12.Jul.01, 27.Ago15.Sep.01, 03-22.Sep.02 and 01-21.Sep.03, including abiotic and biotic information, comparing the results with historical data. The moderate 2002 El Nino was not evidenced, because there were analyzed only temperature; salinity and chlorophyll a at surface and the depth of the termocline was not considered. The highest abundances of chaetognaths in the period 03-22.Sep.02 suggested better general conditions for those organisms, despite of the reigning El Nino. The global analysis of the studies on chaetognaths realized until now in the CPO showed that these organisms are not useful as indicators of anomalous episodes in that area, since they appear indistinctly in normal periods, as during El Nino and La Nina, in very variable densities and distribution; Sagitta minima could be the exception as indicator of cold events. The high density and permanent wide distribution of S. enflata and S. hexaptera demonstrate that these species are the only euritypical ones at the CPO, while the presence of the mesozooplanktonic S. hexaptera, S. serratodentata and K. subtilis, just as the euribatial S. zetesios at the surface evidence upwelling processes in the study area.

Interannual variability in a predator–prey interaction: climate, chaetognaths and copepods in the southeastern Bering Sea

The interaction of the chaetognath Sagitta elegans with the copepod community of the southeast Bering Sea middle shelf was examined in relation to environmental conditions during 1995–1999. Predation impact was estimated for 2 years, 1995 and 1997, using gut content analysis, experimentally derived digestion time (DT) and abundances of chaetognaths and prey. Pseudocalanus concentrations correlated with water temperature and Calanus marshallae with sea ice extent. Sagitta elegans were less abundant but individuals were larger in 1995, when C. marshallae predominated, compared to 1997, when Pseudocalanus and Acartia were the primary prey. Predation by S. elegans removed <1% standing stock day � 1 of Pseudocalanus or C. marshallae in 1995 and 1.7 to 2.3% of Pseudocalanus in 1997. The percent of the copepod community biomass required by chaetognaths was estimated to be <1% in 1995 compared with 8–12% in 1997. Calanus marshallae may be more vulnerable than Pseudocalanus to cumulative predation effects because of its reproductive strategy. The effect of chaetognath predation on the copepod community depends on which copepod species is predominant and its susceptibility to cumulative predation effects, as well as on daily predation impact, both of which varied between years with different climatic conditions.

Seasonal chaetognath abundance and distribution in a tropical estuary (Southeastern, Brazil)

This study focuses on the seasonal variation of the chaetognath species in the Vitória Bay/Passage Channel estuarine system, Espírito Santo, Brazil, in terms of their abundance and distribution. Specimens of chaetognaths were collected between July 1997 and April 1998 at 10 sampling stations, with a cylindrical-conical plankton net of 200 µm mesh and 30 cm mouth, fitted with a mechanical flowmeter. Five chaetognath species were identified: Sagitta enflata, Sagitta decipiens, Sagitta hispida, Sagitta friderici and Sagitta minima. Most of them were distributed in areas of high salinity (e.g. at the stations closest to the outer estuary). The dominant species, S. enflata and S. friderici, were more frequent in the outer estuary where salinities varied from 32 (wet season - summer) to 28 (dry season - winter). S. friderici was the only species found right in the middle of the Passage Channel, at a station close to the main freshwater input into the estuary. Results showed that chaetognaths only enter the estuary due to the tidal effect, and that they are not typical residents of this system. This is to be expected because the group normally inhabits only truly marine regions.