Mónica S. Hoffmeyer

Seasonal succession of Copepoda in the Bahía Blanca estuary

The abundance and species composition of Copepoda with respect to other mesozooplanktonic groups were studied at the harbour of Ingeniero White in the inner zone of the Bahia Blanca estuary, between July 1990 and August 1991. Maximal copepod abundances of 4.7 × 10 m−3 and 4.9 × 10 m−3 were observed in January 1991 and May 1991, respectively. Minimal abundances of 6 m−3 were found in June 1990. Acartia tonsa was present throughout the year with high dominance in summer-autumn (December to May). Eurytemora affinis was present from July to October 1990 (first pulse) and from July to September 1991 (second pulse).

Environmental regulation of the estuarine copepods Acartia tonsa and Eurytemora americana during coexistence period

The seasonal dynamics of Acartia tonsa and the invader Eurytemora americana were analysed in relation to the environmental variability occurring from April to November in the Bahia Blanca Estuary. Twice a month, the abundance of eggs, nauplii, copepodites and adults was examined and some environmental variables were recorded. Multivariate statistics (CCA) was applied to analyse the data of variables. Acartia tonsa eggs and nauplii diminished from April–May and they were almost absent between June and September, although a small larval peak could be detected from the end of July to October. All the stages of this species increased in number through spring. Eurytemora americana was registered as from June and only nauplii larvae were observed, with a peak increase during September. Copepodites and adults were observed as from July, increasing in number until peaking at the end of September. The number of all stages of this species decreased abruptly, the whole population disappearing from the plankton. The A. tonsa developmental stages were most positively correlated with temperature, photoperiod and other light variables whereas those of E. americana showed positive correlations with chlorophyll- a and salinity. The gradients of the main environmental factors likely give rise to a certain niche separation facilitating the coexistence of the two copepod populations within the period studied.

Vulnerability of pejerrey Odontesthes bonariensis populations to climate change in pampean lakes of Argentina

The vulnerability of the pejerrey Odontesthes bonariensis population in Lake Chasicó was assessed under different climate change conditions. During the sampling period, the water temperature was adequate for fish reproduction and to sustain an adequate sex ratio. Climate-driven higher temperatures, however, may severely distort population structure and cause drastic reduction or local extinction of stocks. Lake Chasicó can be classified as eutrophic with clear waters and cyanobacteria that regularly cause fish mortality were identified as Nodularia spumigena and Oscillatoria sp. Global warming may strengthen the effects of eutrophication (e.g. toxic blooms or anoxia). Since many Cyanophyta species tolerate higher temperatures better than other algae, toxic blooms could increase. Furthermore, cyanobacteria have low nutritional value and could decouple the low-diversity food web. Lake Chasicó has currently the salinity optimum (c. 20) for the development of the early life-history stages of O. bonariensis. Climate change, however, is likely to amplify the intensity of droughts or inundations. Floods can endanger O. bonariensis development due to its sub-optimal growth at low salinity and droughts could increase lake salinity and also temperature and nutrient concentration. In order to reduce some of the effects of climate change on the O. bonariensis population in Lake Chasicó, integrated basin management based on an eco-hydrological approach is proposed.

Micro and mesozooplankton composition during winter in Ushuaia and Golondrina Bays (Beagle Channel, Argentina)

Este trabalho analisa o micro e o mesozooplâncton das Baias Ushuaia e Golondrina, constituindo a primeira pesquisa realizada nessas areas sobre estas fracoes do plâncton no inverno (agosto 2004). O numero dos taxa do microzooplâncton e do mesozooplâncton foi mais elevado na Baia Ushuaia do que na Baia Golondrina. Os ciliados aloricados foram dominantes sobre os tintinideos, enquanto que no mesozooplâncton o holoplâncton foi dominante nas duas baias. Ctenocalamus citer, Drepanopus forcipatus e Clausocalamus brevipes foram as especies mais frequentes. No meroplâncton, Halicarnus planatus e Munida gregaria foram as larvas de decapodes mais frequentes em ambos os locais. Os diferentes grupos de estacoes formados em funcao do microzooplâncton e do mesozooplânkton, e detectados na analise de agrupamento, sugerem a influencia de condicoes naturais em cada baia e de diferencas ambientais antropogenicas entre as duas baias.

Impact of a power plant cooling system on copepod and meroplankton survival (Bahía Blanca estuary, Argentina)

The impact of a power plant cooling system in the Bahia Blanca estuary (Argentina) on the survival of target zooplanktonic organisms (copepods and crustacean larvae) and on overall mesozooplankton abundance was evaluated over time. Mortality rates were calculated for juveniles and adults of four key species in the estuary: Acartia tonsa Dana, 1849 and Eurytemora americana Williams, 1906 (native and invading copepods), and larvae of the crab Chasmagnathus granulata Dana, 1851 and the invading cirriped Balanus glandula Darwin, 1854. Mean total mortality values were up to four times higher at the water discharge site than at intake, though for all four species, significant differences were only registered in post-capture mortality. The findings show no evidence of greater larval sensitivity. As expected, the sharpest decrease in overall mesozooplankton abundance was found in areas close to heated water discharge.

Sewage pollution effects on mesozooplankton structure in a shallow temperate estuary

The effects of a sewage effluent with no treatment on the mesozooplankton structure and the environmental quality were evaluated in the Bahía Blanca Estuary, during June to November 1995. The highest values of particulate organic matter, nutrients and specially phosphate, were observed in the effluent discharge zone. In addition, taxa richness, mesozooplankton abundance and Shannon diversity values were lower in the sewage discharge area compared with the less polluted area. Eurytemora americana and Acartia tonsa as well as larvae of Balanus glandula, Neohelice granulata and Spionidae were found in the discharge area with lower densities. These results highlight the importance of sewage effluent effects on mesozooplankton community providing background data to use in other monitoring programmes.

Planktonic food and foraging of Eubalaena australis, on Peninsula Valdés (Argentina) nursery ground

Se reportan observaciones sobre alimentacion de la ballena franca austral y zooplancton disponible, realizadas en Bahia Piramides (Golfo Nuevo). Se compararon datos de plancton, comportamiento trofico de las ballenas y temperaturasalinidad y clorofila-a registrados los dias 10 y 19 de octubre de 2005. El 19/10 la biomasa zooplanctonica fue significativamente mayor, observandose un numero alto de ballenas filtrando en superficie. Esta conducta se observo desde el 17 al 21/10. El evento de forrajeo descrito, confirma que la ballena franca se alimenta en este zona reproductiva de parches de zooplancton con una adecuada composicion y alta densidad, en primavera.

ZOOPLANKTON ASSEMBLAGES FROM A TIDAL CHANNEL IN THE BAHÍA BLANCA ESTUARY, ARGENTINA*

B S T R A C T In this study we investigate for the first time the zooplankton assemblages in a relatively small tidal channel called Bahia del Medio, situated in the middle-outer area of the Bahia Blanca Estuary, South of the Main channel. We focused the study on micro- and mesozooplankton assemblages with emphasis on tintinnids, copepods and meroplankton along the annual cycle 1997-1998. Fifteen species of tintinnids belonging to 6 genera were observed in the study area, the mean total tintinnid abundance fluctuating between 3.4 x 10

Morphometric variables and individual volume of Eurytemora americana and Acartia tonsa females (Copepoda, Calanoida) from the Bahía Blanca estuary, Argentina

Eurytemora americana and Acartia tonsa are the two most important copepods from the Bahía Blanca estuary plankton. For this study, 30 females were selected from recently preserved samples. Prosome length (PL), width (PW) and height (PH), urosome length (UL) and width (UW) measured in each female and subsequently the following ratios PL/PW, PL/PH, PW/PH and PL/UL were determined. Individual volumes for E. americana and A. tonsa females were estimated following the morphometric method in order to obtain individual biomass values. Regression tests were conducted for both species between individual volume and prosome length or width as independent variables, fitting data to a power model. Regression lines were compared through covariance analysis and single pair comparisons in order to evaluate the temporal behavior of the relationships. PL/UL was the best ratio for separating one species from the other. Average individual volume and std. error estimated for E. americana and A. tonsa were 0.0749 (±0.0097) mm3 and 0.0399 (±0.0049) mm3, respectively. Strong differences between both copepods morphometric variable and rate values were observed. A size and volume decrease was detected associated with higher temperatures and less phytoplankton available for food. According to R2 values of regression lines prosome width and prosome length were good volume predictors on different dates for E. americana, however, prosome width was the best predictor on the majority of dates for A. tonsa. Differences in slopes and mean variances found between regression lines demonstrated seasonal variability. General regression equations between volume and PL or PW for each species could be derived.

Seasonal occurrence and hatching success of benthic eggs of calanoid copepods in the Bahía Blanca Estuary, Argentina

Abstract The seasonal abundance and hatching success of calanoid benthic eggs were examined during the coexistence of Acartia tonsa and Eurytemora americana in the Bahía Blanca Estuary (BBE). The dormancy behaviour of eggs extracted from surface sediment layers was tested by simulating seasonal conditions during incubation. There was a greater abundance of benthic eggs of E. americana (≤8.68×106 eggs m−2) than of A. tonsa, the latter consistently showing low egg abundance (≤0.8×106 eggs m−2). The emergence of most E. americana nauplii occurred at 17°C after the experimental warming phase (refractory phase of 280 days), confirming the diapause character of these eggs. The emergence of the first A. tonsa nauplii from initial cold season samples occurred at 9–12°C after 60–70 days of incubation. However, a 180-day delay in hatching at temperatures ≥15°C was also observed in A. tonsa eggs from the last sampling days of the cold season, thus suggesting an intermediate behaviour between diapause and quiescence. The benthic egg bank in the inner zone of the BBE provides a temporary reservoir for both key copepods, thus contributing in their survival.