In situ grazing pressure and diel vertical migration of female Calanus euxinus in the Black Sea

Gut pigment and abundance of the female Calanus euxinus (Hulsemann) weremeasured from several water layers (defined by density values), with3–5 h intervals during 30 h and 21 h at a station in the southwesternBlack Sea in April and in September 1995, respectively. The female C.euxinus was observed to begin migration to the upper phytoplankton-richlayer approximately 3 or 4 hours before the sunset. Only a fraction of thefemale Calanus population (0.2% in April and 3.6% inSeptember) did not migrate but remained at the depth of the oxygen minimumzone during the nighttime. The migrating population was determined to havespent 7.5 h in the euphotic zone in April and 10.5 h in September. Thegrazing rate of female Calanus euxinus was measured from the gut contentdata collected from the layers which contain the euphotic zone. Thepercentage of primary production grazed by the female C. euxinus wascalculated as 14.5% in April and 9.5% in September. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemical composition, respiration and feeding rates of the new alien ctenophore, Beroe ovata, in the Black Sea

We assessed the effects of rotenone on aquatic invertebrate communities by comparing four prairie wetlands treated with rotenone to four control sites. Data collected one week before and three weeks after treatment in the fall of 1998 were paired to assess short-term effects, while data collected in spring 1998 and spring 1999 were paired to assess longer-term effects and recovery rates. Data were collected on 14 taxa of benthic invertebrates collected in Ekman grab samples, and 23 taxa of planktonic-nektonic invertebrates collected in water-column samples. Each data set was analyzed separately with redundancy analysis to assess effects in the two habitats sampled. Significant short-term effects were detected on invertebrates in the water column and abundance of several taxonomic groups declined sharply after treatment. The greatest declines were observed in zooplankton abundance; effects on macroinvertebrates were much less pronounced. Suppression of water-column taxa was short-lived, as significant effects were no longer evident during May 1999. In contrast, no significant short-term effect was evident in the benthic taxa. Our results indicate that fall applications of rotenone may briefly suppress plankton communities, but effects are short-lived. From a fisheries management perspective, fall applications may minimize effects on invertebrate communities and facilitate rapid recovery.     

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.     

Population dynamics, ingestion, growth and reproduction rates of the invader Beroe ovata and its impact on plankton community in Sevastopol Bay, the Black Sea

The impact of the introduced ctenophore Beroe ovata on its preyMnemiopsis leidyi, another invader ctenophore voraciously feedingon mesozooplankton, and consequently on the mesozooplanktoncommunity, was evaluated by undertaking both laboratory andfield studies in the northern Black Sea. Ingestion and growthrates as well as the gross growth efficiency of B. ovata wereestimated from laboratory experiments. The daily ration of ctenophoreswas related to food abundance within a wide range of prey concentrationand never reached saturation. Beroe ovata required high foodrations (not less than 20% of body weight per day) for growth.The abundances, biomasses and population structures of thesetwo introduced ctenophore species were also monitored, alongwith mesozooplankton, in inshore waters of the northern BlackSea (i.e. Sevastopol Bay and adjacent regions) over a periodof 3 years (1999–2001) which is after B. ovata’sarrival. The annual dynamics of the M. leidyi population weresimilar for the last 3 years: very low abundances and biomassvalues were observed during most of the year (unlike the previousyears), with a sudden increase in summer–early autumn,but only for about a 2 month period. The B. ovata bloom duringthe peak M. leidyi biomass resulted in the M. leidyi biomassfalling sharply to extremely low values. The predatory impactof M. leidyi on prey zooplankton was found to be reduced duringthe period of study compared with before.     

Determination of age and growth ofBuccinum undatum L. (Gastropoda) off Douglas,Isle of Man

Age and growth ofBuccinum undatum off Douglas (Isle of Man, U.K.) were studied using four independent methods: length-frequency analyses (LFA), operculum analysis (OA), markrecapture experiment (MRE) and laboratory rearing (LR). Within the 16-month period between February 1989 and June 1990, only the January 1990 sample showed histograms allowing possible age group determination. Among the several length frequency analyses applied to this sample, the growth analysis program MIX calculated thatBuccinum reaches mean lengths of 28.5, 45.8, 59.9, 71.5 and 81.0 mm at the end of its first, second, third, fourth and fifth year, respectively, with an asymptotic length (L_∞) of 123.8 mm and Brody growth coefficient (K) of 0.20. Whilst the results from the LFA were supported by those from the OA and the MRE, growth in the laboratory differed; however, this was expected. Both K and L values found in this study are higher than those in two previous studies undertaken forBuccinum in northeastern England and northern France.     

Estimation of the density ofBuccinum undatum (Gastropoda) off Douglas,Isle of Man

The density of the common whelk (Buccinum undatum L.) off Douglas, Isle of Man, was determined by four methods: (1) pot sampling, (2) diving, (3) mark-recapture experiment, and (4) underwater television. Although the values obtained by these methods were comparable, the last two methods yielded overestimations ofBuccinum density. The results from diving survey and from pot sampling showed a good agreement, indicating that pot sampling can be used to determine the density of the common whelk, provided a good estimate of the pot attraction area is available. The range of whelk density between February 1989 and August 1990 resulting from pot sampling was between 0.08 and 0.38 individuals m⁻². The temporal fluctuations of the whelk densities are discussed in detail.