Seasonal dynamics of Acartia bifilosa and Eurytemora affinis (Copepods: Calanoida) in relation to abiotic factors in the northern Baltic Sea

Distributions in the time and space

Seasonal dynamics of Acartia bifilosa and Eurytemora affinis (Copepods: Calanoida) in relation to abiotic factors in the northern Baltic Sea     

Effects of salinity and temperature on the expression of enzymatic biomarkers in Eurytemora affinis (Calanoida, Copepoda)

In order to establish effective enzymatic biomarkers that could provide in situ early warning of contaminant exposure in estuarine ecosystems, the potential effects of the principal abiotic factors (temperature and salinity) were investigated on common biomarkers, the acetylcholinesterase (AChE) and the glutathione S-transferase (GST) in Eurytemora affinis. Short term salinity stress effects simulated during an experimental tide indicated that enzymatic activities of this species are characterized by maximum expression related to an optimal salinity range (between 5 and 15 psu). Moreover, longer time exposure to various salinity tanks confirmed the effects of this factor on both AChE and GST activities. Therefore, optimal AChE activity was measured at 10 psu, while optimal GST activity was measured at 5 psu. Furthermore, significant effects of temperature were also recorded, particularly for AChE expression (slight effects were measured on GST expression) with an optimal condition at 11 degrees C. These experiments indicated a more pronounced effect of salinity over temperature especially on the AChE expression and confirmed the need to standardize sampling procedures in relation with environmental parameters for biomonitoring studies based on enzymatic analyses.     

Drift-ice and under-ice water communities in the Gulf of Bothnia (Baltic Sea)

The algal, protozoan and metazoan communities within different drift-ice types (newly formed, pancake and rafted ice) and in under-ice water were studied in the Gulf of Bothnia in March 2006. In ice, diatoms together with unidentified flagellates dominated the algal biomass (226 ± 154 μg ww l⁻¹) and rotifers the metazoan and protozoan biomass (32 ± 25 μg ww l⁻¹). The under-ice water communities were dominated by flagellates and ciliates, which resulted in lower biomasses (97 ± 25 and 21 ± 14 μg ww l⁻¹, respectively). The under-ice water and newly formed ice separated from all other samples to their own cluster in hierarchical cluster analysis. The most important discriminating factors, according to discriminant analysis, were chlorophyll-a, phosphate and silicate. The under-ice water/newly formed ice cluster was characterized by high nutrient and low chlorophyll-a values, while the opposite held true for the ice cluster. Increasing trends in chlorophyll-a concentration and biomass were observed with increasing ice thickness. Within the thick ice columns (>40 cm), the highest chlorophyll-a concentrations (6.6–22.2 μg l⁻¹) were in the bottom layers indicating photoacclimation of the sympagic community. The ice algal biomass showed additional peaks in the centric diatom-dominated surface layers coinciding with the highest photosynthetic efficiencies [0.019–0.032 μg C (μg Chl-a ⁻¹ h⁻¹) (μE m⁻² s⁻¹)⁻¹] and maximum photosynthetic capacities [0.43-1.29 μg C (μg Chl-a ⁻¹ h⁻¹)]. Rafting and snow-ice formation, determined from thin sections and stable oxygen isotopic composition, strongly influenced the physical, chemical and biological properties of the ice. Snow-ice formation provided the surface layers with nutrients and possibly habitable space, which seemed to have favored centric diatoms in our study.     

Daily egg production ofTemora longicornis (Copepoda,Calanoida) during winter and early spring in the Marsdiep (Southern North Sea)

Mature females of the copepodTemora longicornis from the Marsdiep tidal inlet were incubated at ambient water temperature during the winter of 1989/1990 to estimate their egg production during 24 hours. Throughout winter this production remained at a level of 0 to 10 eggs per female. In March a steep rise to 60 to 80 eggs coincided with an equally steep rise in chlorophyll-a from 4 to 20 g.l^–1. Excess food given during incubation tended to increase the observed production rate only in October at relatively high temperature. Additive effects of rising food level and temperature on daily egg production were found in spring. In March and April 1988 and 1990 the observed maximum daily egg production per female increased from about 20 at 5°C and 5 g.l^–1 chlorophyll to 70 at almost 10°C and 20 g.l^–1. The egg production in winter gives a prebloom potential to maintain a stock of premature stages ready to utilize the algal spring bloom. Early spawning before April can also contribute to the development of the abundance peak of adults in May–June. Due to growth rates increasing with temperature in the months March and April, most of the adults at their peak density may have hatched from eggs before the period of maximum egg production rate at the end of April. The exceptionally high production rate observed at the end of March may be related to a developingPhaeocystis bloom.     

Variations in the embryonic development times and hatching success of three Boeckella species (Copepoda: Calanoida)

Embryonic development times of Boeckella triarticulata, B. dilatata and B. hamata are compared. Two species, B. triarticulata and B. hamata had widely varying embryonic development times. Variable development times often resulted in greater hatching success.     

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     

A new species of Acartia (Copepoda, Calanoida) from the Yellow Sea

A new species (Acartia hongi) of Acartia (family Acartiidae)predominating in the Korean coastal waters of the Yellow Seais described. This species closely resembles Acartia bifilosa,from which it can be easily distinguished by the co-occurrenceof the following morphological characters: the absence of rostralfilaments in both genders; the terminal spine on the femalefifth leg toothed only on the inner side; the first exopodalsegment of the male right fifth leg with a long seta; the distalsegment of the male left fifth leg with a rod-like appendage.Its distribution was discussed in comparison with two siblingspecies, Acartia omorii and Acartia hudsonica.     

Some reflections on the structure of the mandibleplate ofEurytemora affinis (Copepoda,Calanoida)

The mandibleplates of several specimen of the copepodEurytemora affinis collected from the Westerschelde and the Gironde estuary were studied. Using the Edge index (ITOH, 1970) this species could be classified as an omnivore. Pictures obtained by light and scanning electron microscopy reveal that the mandible teeth ofE. affinis have a molar-like structure, which likely changes in functional morphology with developmental stage, sex and age of the adults. Based on these observations, the desirability to consider the three dimensional structure of copepod mandibles for feeding mode classification is discussed.Communication no. 540 of the Delta Institute for Hydrobiological Research, Vierstraat 28, 4401 EA Yerseke, The Netherlands.     

A new species of Bradyidius (Copepoda, Calanoida) from the Gulf of Carpentaria, Australia

A new species of copepod belonging to the family Aetideidae,Bradyidius slyliformis sp. nov. is described and figured. Therelationship of this species to the other 12 members of thegenus is discussed. ¹Present address: 121 Jalan Athinahapan Dua, Taman Tun Dr Ismail,60000 Kuala Lumpur, Malaysia     

Development of Acartia bifilosa (Copepoda: Calanoida) eggs in the northern Baltic Sea with specialreference to dormancy

Development of eggs produced by Acartia bifilosa in summer andautumn was studied in the northern Baltic Sea. Resting eggsof the species have previously been found in sediments, andthe aim of this study was to reveal the type of dormancy inthe eggs. Eggs were incubated at temperatures ranging from 1.5to 18°C. The effect of continuous darkness on hatching wasalso tested. Hatching success in the experiments varied between56 and 97%. Egg development was similar in summer and autumn,indicating that A. bifilosa does not produce diapause eggs inthe area. Furthermore, dormancy was not induced at any of thetemperatures tested, nor by darkness. Results are compared withthose of Castro-Longoria and Williams (Castro-Longoria and Williams,1999b, J. Plankton Res., 21, 65–84) who studied A. bifilosain the English Channel, where both production of diapause eggsand arrest of development in the subitaneous eggs at low temperaturesoccurred. The possible causes of the difference in dormancystrategies in the two areas are discussed.     

Seasonal variations in carbon and nitrogen content of Acartia clausi (Copepoda,Calanoida) in the Gulf of Trieste (Northern Adriatic Sea)

A yearly study was made on total and cephalothorax lengths, dry weight, carbon and nitrogen contents and C:N ratio of the species Acartia clausi (Copepoda, Calanoida) which is always abundant, particularly in spring, in the net zooplankton community of the Gulf of Trieste (Northern Adriatic Sea). The samples of net zooplankton were carried out every month at the permanent station located 200 m offshore and preserved for about one year in 4% buffered formalin to insure weight loss stabilization. A. clausi was separated from other zooplankton specimens; washed and dried. For each monthly sample the average total and cephalothorax lengths and the average dry weight of adult organisms of A. clausi were measured. The mean carbon and nitrogen contents, as a percentage of dry weight, were determined by using a CHN analyser. Linear regression models were computed on the log-transformed data in order to check the relationships between dry weight, total and cephalothorax lengths, carbon and nitrogen contents and C:N ratio of A. clausi. The best of the various established correlations, was between dry weight and carbon content. Our study pinpointed a high seasonal variability of the C:N ratio, mainly due to seasonal fluctuations in the nitrogen content.     

Egg production rates and stage-specific development times of Clausocalanus furcatus (Copepoda, Calanoida) in the northern Gulf of Mexico

The calanoid copepod Clausocalanus furcatus is widely distributedin tropical and subtropical oceans, yet knowledge of its reproductionand development remains limited. We estimated egg productionrates and stage-specific development times from laboratory incubationsand field samples collected at a petroleum platform in the northernGulf of Mexico. Egg production experiments were performed in2002 and 2003 at a laboratory on the platform using freshlycollected female copepods. Median stage development durationswere also measured during the incubations. Logistic regressionmodels were used to estimate median stage durations. The mediandevelopment times of Clausocalanus furcatus from hatching toadulthood ranged from 13 to 20 days. Field samples were collectedfrom waters beneath the platform at 12-h intervals using a 30-LNiskin water bottle during March–April and May–June2003, and the Edmonson egg ratio method was used to estimatethe mean in situ egg production rate. This field-derived meanegg production rate was significantly lower than the 12.08 eggsfemale^–1 day^–1 measured from incubation experiments,and this difference may reflect high in situ egg mortality.In situ egg production rates also showed high variability andwere almost five times higher during March–April comparedto May–June.     

Resting egg production and oviducal cycling in two sympatric species of alpine diaptomids (Copepoda: Calanoida) in relation to temperature and food availability

Resting egg production and oviducal cycling were investigatedfor the calanoid copepods Arctodiaptomus alpinus and Acanthodiaptomusdenticornis both in the laboratory and in a small karstic alpinelake by making a census of the number of eggs produced and theproportion of females in each of four morphologically distinguishedreproductive conditions each day in the laboratory or duringa 2–3 week period in lake enclosures. In the laboratory,individuals were maintained on a mixed diet of natural phytoplanktonat constant temperatures of 4, 10, 15 and 20C, respectively.Both species differed considerably in their temperature requirementsfor reproduction. Lifetime fecundity was highest at 10C in A.alpinusand at 20C in A.denticornis, with up to 327 eggs ^–1 spawnedin the former and up to 582 eggs ^–1 in the latter species.Unfavorable temperatures were further reflected in an increasein egg mortality and the allocation of time spent in a post-reproductivephase, as well as in a decrease of longevity. Increasing temperaturesenhanced egg production rates due to decreasing clutch productionperiods, although clutch size was negatively correlated withtemperature. Maximum rates reached 5.88 and 7.98 eggs ^–1day^–1 in the laboratory, and 0.73 and 0.55 eggs ^–1day^–1 in enclosures in A.alpinus and A.denticornis, respectively.Egg production rates and clutch size were clearly governed bynutritional conditions in the lake, but were less affected byfood supply in the laboratory. Here, rates of egg productionwere adapted to improving food supply by increasing the frequencyof spawning events, rather than the number of eggs per clutch.No correlation was found between female body size and reproductiveparameters in the laboratory. A very low proportion of totalclutch production resulted in clutches composed of subitaneouseggs, i.e. 0.14% in A.denticornis and 1.20% in A.alpinus. Oviducalphase duration allocations indicate that there exists a temperatureoptimum for gamete maturation.     

Mesaiokeras hurei n. sp. (Copepoda, Calanoida, Mesaiokeratidae) from the Adriatic Sea

A new species of Mesaiokeras is described from the hyperbenthosof Veliko Jezero, a marine lake on the island of Mljet, Croatia.This is the first species of this genus and family (Mesaiokeratidae)reported from the Adriatic and Mediterranean seas. It is characterizedby small body length (an average 361 µm for females and337 µm for males), the low number of free segments (21)and the armature of the antennules, very long spine (a₂) ofexopodite I of female right leg 4, the presence of long processeson the ventroposterior area of the female genital double-somite,the form and surface ornamentation of the genital area, theform and ornamentation of male and female leg 5, and the formand armature of caudal rami. This species was found in shallowwater with very stable hydrographic conditions and at a populationdensity from 1 to 26 ind. m^-3.     

Temporal variability of abundance and reproductive traits of Centropages kroyeri (Calanoida; Copepoda) in Bizerte Channel (SW Mediterranean Sea, Tunisia)

Abundance and reproductive traits (spawning female, egg production, egg hatching success) and naupliar survival of the dominant calanoid copepod Centropages kroyeri were investigated in relation to biotic (chlorophyll a, phytoplankton and microplankton) and abiotic (temperature and salinity) parameters at a monitoring station located in Bizerte Channel, northern Tunisia. The monitoring was carried out between July 2004 and December 2004, according to two temporal sampling strategies. First, during July, the supposed maximum abundance period of C. kroyeri, daily sampling was conducted. Thereafter, sampling was carried out weekly until the end of the sampling period.The temporal variability of the environmental factors in relation to the reproductive traits of C. kroyeri revealed, first, that temperature is important in controlling the reproduction and dynamics of this species, and second, that the quantity and quality of food available in the field also strongly influence its reproduction. On several sampling dates, the output of certain reproductive traits such as hatching success after 24 h was very weak. It is possible that C. kroyeri produced resting eggs during the study period. Nerveless, the analysis of the phytoplankton during these periods showed that low reproductive activity may have been caused by low phytoplankton quantity and/or by the dominant presence of certain diatom species that are supposed to have an inhibitory effect on the reproductive traits of C. kroyeri, especially on egg hatching success.Among the considered external parameters, temperature and food quality and availability seem to be the most important factors for the reproduction and dynamics of C. kroyeri. Under these biotic and abiotic field conditions, this copepod seems able to modulate the abundance of its population while controlling its reproduction.     

Growth and development of Metridia pacifica (Copepoda: Calanoida) in the northern Gulf of Alaska

Juvenile growth and development rates for Metridia pacifica,one of the dominant larger copepods in the subarctic Pacific,were investigated from March through October of 2001–2004in the northern Gulf of Alaska. The relationship between prosomelength (PL, µm) and dry weight (DW, µg) was determined:log₁₀ DW = 3.29 x log₁₀ PL – 8.75. The stage durationsof copepodites ranged from 3 to 52.5 days but were 8–15days under optimal condition. Seasonally, growth rates increasedfrom March to October and typically ranged between 0.004 and0.285 day^–1, averaging 0.114 ± 0.007 day^–1(mean ± SE). After standardization to 5°C (Q₁₀ of2.7), growth rates averaged 0.083 ± 0.005 day^–1and were significantly correlated to chlorophyll a, with saturatedgrowth rates of 0.149 day^–1 for C1–C3, 0.102 day^–1for C4–C5 and 0.136 day^–1 for all stages combined.Measured juvenile growth rates were comparable with specificegg production rates in this species. The comparisons of ourrates in this study with those predicted by the global modelsof copepod growth rates suggested that further refinement ofthese models is required.     

Effect of temperature and food concentration on post-embryonic development, egg production and adult body size of calanoid copepod Eurytemora affinis

Post-embryonic development time, egg production rate and adultbody size of calanoid copepod Eurytemora affinis from Lake Ohnuma,Japan, were determined under six temperature-food conditions(10³,5 x 10³, 10⁴ and 5 x 10⁴ cells ml^–1 at 15°C,5 x 10⁴ cells ml^–1 at 10and20°C) in the laboratory.The measured parameters varied with both temperature and foodconcentration. Development time from hatching to adult femalewas 9.2, 11.4 and 22.8 days at 20, 15 and 10°C respectively,at the highest food concentration. The males developed to adultat one to two days earlier than the females. An effect of foodshortage on development time occurred at the lowest food concentration.This development time was 24.8 days even at 15°C, beingtwice as long as that at the highest food concentration. Prosomelength of these food-limited females was approximately 75% ofwell-fed ones, which reduced by only 10% with increasing temperaturefrom 10 to 20°C. Clutch size (C, eggs clutch^–1) ofwell-fed individuals depended on prosome length of the adultfemale (L, mm), and was expressed as an equation: C = 65.2 L³⁸³. Clutch size of individuals reared at less than 10⁴ cellsml^–1, however, mostly laid below the estimated curve,especially at the lowest food concentration being only 10% ofthat at the highest food concentration. These results suggestthat food availability is a more important factor affectingpopulation growth of E.affinis in Lake Ohnuma than variationof temperature.     

The relative importance of egg production rate, hatching success, hatching duration and egg sinking in population recruitment of two species of marine copepods

The disappearance of spawned copepod eggs can, at times, approach100% day^–1 and may be a bottleneck to population recruitmentof marine copepods. We examined the egg production rate andegg hatching success of Centropages hamatus and Temora longicomis(Copepoda: Calanoida) on natural diets, and the role of delayedhatching combined with high sinking rates in removing theireggs from the water column. Cumulative hatching success withinIS days was consistently high from March to June 1996:     

Community analysis of sublittoral Harpacticoida (Copepoda,Crustacea) in the western Baltic Sea

This study compares two stations each sampled at the Øresund (Denmark) and the German coast (western Baltic Sea), with respect to sublittoral Harpacticoida. The associations were found to differ considerably in diversity, species numbers, evenness and also in the species composition. At the two northern stations, mainly polyhaline species were found and they showed the highest species and individual numbers as well as the most even species distribution. At the two southern stations, mainly pleiomesohaline species were found and they were dominated by one species that was very resistant to eutrophication, so that they showed an uneven species distribution, but also had lower individual numbers and thus lower diversity. Some samples at the German coast were found to have an extremely low harpacticoid number, which was attributed to the severe oxygen deficiency that occurred in autumn 2002 in the western Baltic Sea. This can also be considered to be the main reason for the low species diversity of the southern stations.     

Morphological differences between subitaneous and diapause eggs of Boeckella triarticulata (Copepoda: Calanoida)

1. Many calanoid copepods produce subitaneous eggs that hatch immediately and diapause eggs that remain dormant for long periods of time. Boeckella triarticulata (Copepoda: Calanoida) produces diapause eggs that hatch asynchronously over an extended period of time and appear identical under light microscopy to those that hatch immediately.
2. We compared the internal morphology of subitaneous and diapause eggs of B. triarticulata that had been prepared for transmission electron microscopy by four methods.
3. Subitaneous eggs had a thin, single-layered shell whereas diapause eggs had a thick, three-layered shell that appeared consistent over 3 months of diapause. Boeckella triarticulata appears to enter diapause at the multicellular stage.
4. Consistent morphological differences between subitaneous and diapause eggs of B. triarticulata suggest that these two egg types also have distinct differences in physiology, and are not merely extremes of a continuum from subitaneous to diapause eggs.