Recent study of Mnemiopsis leidyi in Ukranian waters of the Black Sea.

This study aimed to estimate predatory impact of two new alien ctenophores Mnemiopsis leidyi and Beroe ovata on planktonic community in inshore waters of the Black Sea.

The research tasks were:

• study annual dynamics of abundance, biomass and population structure of M. leidyi (1995–1996) and both M. leidyi and B. ovata in Sevastopol Bay and adjacent water regions (1999–2000).

• measure ingestion and growth rates of both ctenophore species and the effects of food concentration and body weight on these parameters in the laboratory experiments

• quantify the B. ovata predatory impact on M. leidyi population.

The population dynamics, abundance and biomass of the ctenophore M. leidy was studied in Sevastopol Bay during 2 years (from January 1995 to March 1996 when B. ovata was absent and from September 1999 to March 2001 after B. ovata arrived). Sampling locations are shown in Fig.1.

Figure 1. Sampling locations 1-11 in Sevastopol Bay and adjacent water area

The ctenophores achieved maximum abundance (3000 ind m^–2)in July and maximum biomass (600 g m^–2) in August in 1995 ( Fig.2,a,b ). The young specimens (<10 mm) contributed for that period as much as 90% to the total abundance of the population. The reproduction continued until the end of October but the biomass decreased due to elimination of the over-wintered animals. Maximum wet weight of M. leidyi in population appeared to be before the reproduction (Fig.2,c). During the year, the energy content of the animals of different size groups in the population (1–15 mm) varied but there was no correlation either with the size of the ctenophores or with the season (Fig.2, d).

Figure 2. Abundance (a), biomass (b), and mean wet weight (c) of M.leidyi in the Sevastopol Bay in 1995-1996, (d) – 1 energy content of M.leidyi, 2 – biomass of fodder zooplankton

The maximum specific growth rate was observed during the period of active reproduction of adult animals and intensive growth of young ctenophores (August–September). For the subsequent months this value decreased(Table 1).

Table 1. Specific growth rate of M. leidyi population (µ day^-1 ) in Sevastopol Bay

Note: W₀ is initial wet weight; W_t is final wet weight, g

The growth, food consumption and the clearance rates were measured in the laboratory under two food concentrations (Acartia clausi and Moina micrura; 60 and 100 specimens per liter, 0.35 and 0.60 mg dry weight/l.) Both concentrations were sufficient for the growth of small animals with a dry weight less than 20 mg; however, these concentrations were insufficient for the growth of larger ctenophores (Table 2) . At a concentration of 0.35 mg/l, the maximum specific growth rate as well as maximum gross growth efficiency occurred in the smallest animals (0.15 mg of dry weight) and decreased with increasing of ctenophore weight. The daily rations in small ctenophores varied from 28 to 74% of the body energy content. At these food conditions the specific daily ration of animals > 20 mg was lower (5-15% of the body energy content), but the energy consumed appeared to be insufficient for growth: the ctenophores daily lost about 10% of their body weight.

Table 2. Food consumption and growth of M. leidyi at two food concentrations (Acartia clausi and Moina micrura) in the experiments

Specific growth rate of Mnemiopsis leidyi population (m ) in Sevastopol Bay

Note: W₀– initial wet weight; W_t –final wet weight (g)

The correlation was evident between the specific clearance rate values obtained at both concentrations and the weight of ctenophores (or the energy content). ( Fig.3).

Figure 3. Relationship between clearance rate and body weight of M.leidyi at different food concentrations 1 – 0.35; 2 – 0.60 mg of dry weight per l

On the basis of the results of the field observations and laboratory experiments the predator impact of M. leidyi population on zooplankton in Sevastopol Bay was estimated (Fig.4).

Figure 4. Annual dynamics of grazing impact of M. leidyi on zooplankton biomass in Sevastopol Bay in 1995–1996

During the year ctenophores grazed daily from < 1 to 35% of zooplankton biomass. In winter ctenophore biomass was low and the rations as well as daily grazing values were also low (Table 3). In summer and autumn the biomass of zooplankton increased by a factor of 5–10; at the same time, the maximum development of the ctenophore population occurred. The daily grazing value increased by 21 and 9% of zooplankton biomass. The ctenophore population could meet the minimum food requirements in Sevastopol Bay for 3–10 days only. The ctenophores experienced a lack of food, especially during the summer and autumn. This lack would have resulted in subsequent decrease of M. leidyi population.

Table 3. Grazing of zooplankton biomass by M.leidyi population in Sevastopol Bay during different seasons in 1995–1996

The study carried out in Crimean coastal waters in spring 1999 (March–April ) showed that M. leidyi biomass as well as the percentage of this species in gelatinous macroplankton total biomass dropped in compare to spring 1995 (Fig.5).

Figure 5. The percentage of different species in the gelatinous zooplankton biomass in March–April

In August 1999 new alien ctenophore B. ovata has appeared in Sevastopol Bay and adjacent water regions. As previous studies showed this species feed on other ctenophores, in case of the Black Sea, almost exclusively on M. leidyi.

We attempted to quantify the predatory impact of B. ovata on M. leidyi population by studying feeding and growth rates of Beroe in the laboratory along with abundance, biomass and population structure of both ctenophore species from the shallow waters of the Black Sea.

The Beroe was available in plankton of Sevastopol Bay during 3 months (September–November) in 1999 and 2000. It appeared in the moment of maximum M.leidyi biomass. (Fig. 6 a,b) . Dynamics of M. leidyi population for these years was similar to that in 1995. From winter to beginning of summer (mid July) M. leidyi abundance was very low; the large animals of 20-50 mm were the most numerous. M. leidyi over-wintered began reproduction at the end of August or beginning of September in 2000; in 1999 it was earlier. Reproduction continued until October- beginning of November. Juvenile ctenophores < 10mm comprised about 60-90% population abundance at this time in both years. Later, in November-December animals of 15-35 mm were the most numerous. In 2000 maximum biomass and maximum abundance (860 ind m^–2, 210 g m^–2) were observed at the beginning of September. Maximum wet weight of M. leidyi in population in 2000 was much higher (near 20 g) of that in 1995 (4 g)(Fig.6c).

Figure 6. Abundance (a), biomass (b), wet weight (c) of M. leidyi and B. ovata in 1999–2001

In 2000 after B. ovata appearance average values for summer – autumn period of M. leidyi abundance (143±302 ind/m²) reduced8 times and biomass (50±82 g/m² )5 times in compare with 1995 (1100±920 ind/m² and 230±173 g/m²). Size spectrum of animals was wider in 2000: maximum length was 35–40 mm against 15 mm in 1995

Abundance of M. leidyi has dropped sharply when Beroe appeared in mid September (later than it was in 1999) and remained at low level till December. In compare with 1995 when M. leidyi abundance declined three times in relation to maximum value due to natural mortality or carrying out to sea, in 2000 it decreased more than 2 orders of values due to additional predatory impact of Beroe .

In 2000 B. ovata has appeared firstly in mid September and the population was made up of juvenile ctenophores less than 10 mm. To beginning of November ( about 2 months later) large animals (30-50 mm) in the population were the most abundant. To mid November a share of the largest ctenophores (50-70 mm) increased and relative abundance of small ones (10-30 mm) dropped. At this time maximum biomass as well as maximum weight of animal in the population was observed ( Fig 6, b, c). To mid December Beroe disappeared from plankton..

In laboratory experiments the ingestion rate of juvenile and adult B. ovata is proportional to food concentration over an extremely wide range of prey concentration up to the highest experimental densities used (Fig.7a,b). The daily ration ranged from 5% to 330% of body wet weight.

Figure 7. Effect of food concentration on daily ration injuvenile (a) and adult (b) B. ovata

Specific ration value ( it is a function of food concentration and animal body weight ) is an important factor that affects on growth rate. Fig. 8 shows that at specific ration less than 20% of body weight the ctenophores growth stopped. Food concentration 1.7 g/l was sufficient for growth of Beroe less than 20 g, but larger ctenophores need higher food concentrations for growth.

The daily ration of the B. ovata in the field and its predatory impact on the M. leidyi population were assessed via ingestion and growth rates from laboratory experiments.

Figure 8. Effect of specific ration on growth rate in adult B. ovata

If we suggest that specific ration of B. ovata was 20% of their weight (as it was in the experiments) and 20% of the population had any food in their guts the grazing impact would be as much as 10- 35% of M. leidyi population in a day. A comparison of maximum daily specific growth rate of the M. leidyi population that occurs during the period of intensive growth and reproduction in Sevastopol Bay (9.3%) with predatory impact of B. ovata on population of M. leidyi (10-35%) showed that in inshore waters of the Black Sea the new alien population may control abundance of their prey population. Sharp decrease of abundance and biomass of M. leidyi during the period when B. ovata occurs in the bay testify that B. ovata population does control the M. leidyi population in Sevastopol Bay.

It is very important that predatory impact of M. leidyi population on fodder zooplankton in 1999-2000 was very low: only during August-September grazing rate of M. leidyi population was about 6-8% of zooplankton biomass daily, during another months it was less then 1%.

Conclusions.

In 2000 after B. ovata appearance average values for summer – autumn period of M. leidyi abundance (143±302 ind/m²) reduced8 times and biomass (50±82 g/m² )5 times in compare with 1995 (1100±920 ind/m² and 230±173 g/m²). Size spectrum of animals was wider in 2000: maximum length was 35–40 mm against 15 mm in 1995.

Predatory impact of M. leidyi population on fodder zooplankton in 2000 is low: only during August-September predatory rate was 6-8 % of zooplankton biomass daily; during another months it was less than 1%.

Predation by the ctenophore B. ovata (10-35% of M. leidyi biomass daily) is an important factor controlling the M. leidyi population in the inshore waters of the Black Sea.