Long-term changes in the total development time of Copepoda species occurring in large numbers in the Southern Baltic Sea – numerical calculations

With regard to the above data, the development times at two stages of ontogenesis (nauplius and copepodid) as well as the total generation time of the studied species were described by mathematical relationships taking into account the effect of marine environmental parameters: temperature T, salinity S, and concentration of food. Food. The effect of temperature on the development time was described by the ft function (eq (1) for Acartia spp., eq (6) for Temora longicornis and eq (11) for Pseudocalanus sp.), as a parabolic threshold function where the development time increases at a temperature above the optimum value (T > T0), as a result of physiological stress. The development time D of the studied species, as a function of three variables: concentration of food. Food, temperature T and salinity S, allowing for the minimum development time Dmin was described by the equation:

where D _min is the minimum value of the development time, for which the growth rate of an individual is not limited by food availability, i.e. Food=maxFood. It has been assumed that D min = f (T) is a function of temperature defined at salinity of 35 PSU. The value of R is an additional time required for the development in the conditions below the optimum, i.e. Food < maxFood and is defined as a function of temperature and concentration of food R = f (T, Food). The functions fs and fs (i.e. temperature and salinity, respectively) were defined arbitrarily, based on the available data, as functions determining and reducing the development of the studied species.

The following quantitative expressions were used for numerical calculations to describe the effect of the aforementioned environmental parameters (Food, T and S) on the development time of the studied species at the nauplius and copepodid stage as well as the total generation time:

Acartia spp.

for the naupliar stage:

for the copepodid stage:

for the total generation:

Temora longicornis

fs = 2 #8211; (1 #8211; exp (0.5 (S #8211; 2))) (7)

for the naupliar stage:

for the copepodid stage:

for the total generation:

Pseudocalanus sp.

for the naupliar stage:

for the copepodid stage:

for the total generation:

Respectively 91%, 83% and 86% of the D values for Acartia spp, Temora longicornis and Pseudocalanus sp, calculated from the equations (eq. 3, 4, 5 for Acartia spp., eq. 8, 9, 10 for Temora longicornis, eq. 13, 14, 15 for Pseudocalanus sp.) and being a function of food concentration and temperature at 30 PSU, are within the range of values reported by Klein Breteler et al. (1995), Klein Breteler and Gonzalez (1986) and Klein Breteler and Schogt (1994). On the basis of data from the papers by Ciszewski and Witek (1977) and Witek (1995) for Acartia bifilosa and Pseudocalanus elongatus, the calculated value of the development time D is in the range of 81-93% (detailed description of equations will be presented in another paper).

Numerical simulations were performed on the basis of the assumptions of the scenarios regarding the climate changes #8220;Greenhouse Gas Emission#8221; for the years 2000-2100 (Hollweg et al. 2008). Scenarios A1B and B1 were taken into account in the calculations for S1 and S2 based on the study by Neumannn (2010).

Assumptions for numerical simulations:

S1:

a) S1a:

- temperature T rise from by 2.2^oC (autumn, winter) and 4 ^oC (summer)

S2:

c) S2a:

- temperature T rise by 0.2oC (autumn, winter) and 2oC (summer)

Annual distributions of environmental parameters, i.e. temperature T, salinity S and food concentration Food were used as input data to determine the duration of ontogenetic stages (nauplius and copepodid) as well as the total generation time of the studied Copepoda species

Variables T, S and Food (as Food = Phyt + Zoop + DetrP) (Fig. 1) were derived from the 3D CEMBS ecosystem model for station P1 (Gdańsk Deep) in the Southern Baltic for 2010 (Dzierzbicka-Glowacka et al. 2012, 2013a, b; website www.cembs.pl).

Figure 1

The structure (i.e. shape and character) of the obtained curves presenting the variability in the annual cycle of the minimum D min and the total development time DT (from eggs to adult individuals) of the studied species Acartia spp., Temora longicornis and Pseudocalanus sp., divided into stages (nauplii N min and NT, copepodid C min and CT) and the total generation time D _min and DT, is similar in all the examined cases. Differences occur only in the values.

It appears from the performed calculations that environmental conditions for the development of Acartia spp. were much better in both examined cases compared to the initial condition (in terms of food concentration Food and temperature T), and they were more suitable in the case of scenario A1B than in B1.

In the case of scenario A1B, the temperature increase is greater than in B1 by ca. 2 ^oC, which gives effective results in the calculations. In all the examined stages (nauplii NT, copepodid CT and the total development DT), it is clear that the minimum development time D _min for the nauplius, expressed as a function of temperature at the maximum concentration of food, is shorter by ~ 8 days in A1B (~ 5 days in B1) in the winter/spring season, and by ~ 3 days in A1B (~ 1.5 days w B1) in summer, as compared to the initial state. Graphs for the copepodid show similar values. For the total generation time, the reduction in the D _min value is twice as large compared to the nauplii stage and is ~ 17 days in A1B (~ 10 days in B1) in winter (Fig. 2).

Figure 2

Considering the total development time, in both cases (A1B and B1), a shorter duration of ontogenetic stages was observed in scenario A1B compared to B1. The differences are significant in relation to initial values, especially in winter: ~ 35 days #8211; NT, 50 days #8211; CT and 105 days for DT in A1B (S1b), and for scenario B1 (S2b): 30, 50 and 90 days, respectively (Fig. 2).

As evidenced by the obtained results, the temperature and concentration of food have a major influence on the development of Acartia spp. at all stages of ontogenesis and they determine the development of this species. Large differences in the development of this species are observed in winter and early spring when an increase in temperature and the amount of food has a significant influence on the development of a given individual, as well as in summer when the temperature is the main determining factor. The obtained numerical results indicate that the number of Acartia spp. populations will increase in the studied region by at least one or two populations, and the growth rate at each ontogenetic stage will be higher and the first population may be observed in the earlier period of the year.

The calculations performed suggest that in most cases, environmental conditions for the development of Temora longicornis were slightly better in scenario A1B compared to B1.

At different stages of Temora longicornis ontogeny (nauplius NT, copepodid CT, and the total development time DT), the minimum development time D _min is shorter compared to the initial state, only in scenario A1B. Both for the nauplius and copepodid, it is shorter by ~ 5 days in the winter-spring season, and by ~ 1-2 days in summer. In the case of the total development time DT, a decrease in D _min values is doubled compared to D _min at the nauplius stage (Fig. 3).

Figure 3

In both cases (A1B and B1), the total development time DT of Temora longicornis is shorter in relation to the initial values, and the values are lower in scenario A1B compared to B1. These differences in winter are ~ 25 days #8211; NT, 15 days #8211; CT and 60 days for DT in A1B, and for scenario B1 (S2b): 10, 5 and 35 days, respectively. In summer, a decrease in the duration of individual stages is insignificant (Fig. 3).

It appears from the results that (as in the case of Acartia spp.) temperature and food concentration are factors determining the development of Temora longicornis at all stages of ontogenesis. In accordance with the accepted function fs, salinity significantly affects the development of Temora longicornis and slightly reduces the effect of temperature. The differences in the development of the examined individual are observed in winter and early spring when an increase in temperature and the amount of food are of crucial importance. According to the obtained numerical results, the number of Temora longicornis populations may increase in the studied region by half or one population. The development of individuals representing this species will be faster in the winter-spring season and the first population in a given year may occur earlier.

Based on the obtained results, it appears that worse environmental conditions for the development of Pseudocalanus sp. individuals occurred in both examined cases, and they were less favorable in scenario A1B compared to scenario B1, except for spring.

At different stages of Pseudocalanus sp. ontogeny (nauplius NT, copepodid CT and the total generation time DT), the minimum development time D _min is longer in both cases compared to the initial state. These values for the copepodid are ~ 10 days #8211; in A1B and ~ 15 days #8211; in B1 in the winter-spring season, and ~ 7 days #8211; A1B and 4 days #8211; B1 in summer. For the total generation time, Dmin values increased two times in relation to the copepodid stage. It should be noted, however, that the conditions for the development of Pseudocalanus sp. in summer are less favorable in A1B than in B1. This is due to a greater temperature rise in the case of A1B (Fig. 4).

Figure 4

There are considerable differences in the total development time D in relation to the initial values, especially in winter and summer. They are on average ~ 10 days #8211; CT and 30 days #8211; DT in A1B and B1 in winter; and in summer: 7 days #8211; CT and 20 days #8211; DT in B1 and 35 days #8211; CT and 80 days #8211; DT in A1B, respectively. In summer, an increase in the development time D for scenario A1B is significant and almost twofold in relation to the initial values (Fig. 4).

On the basis of the obtained results, we can conclude that the temperature and salinity are parameters that significantly affect the development of Pseudocalanus sp. at all stages of ontogenesis. Salinity, as a masking factor according to the adopted function fs, has the major influence on the development of the studied species in winter, and intensifies the effect of temperature in summer. Significant differences are observed in winter and spring when despite an increased temperature compared to the initial state, ontogeny is inhibited due to reduced salinity. This leads to an increased duration of subsequent stages of ontogenesis in both scenarios. Numerical simulations also show that salinity has a greater effect on the ontogeny during this period than temperature. In summer, both salinity and temperature reduce the development of this species, causing an increase in the duration of different ontogeny stages. This situation is induced by i) a decrease in salinity which has a significant effect on the development of Pseudocalanus sp. causing a reduction in the growth rate, as well as by ii) an increase in temperature above the optimum value To, which in this period has a negative impact (like salinity) on the development of this species.

The presented results of calculations evidence that in the Bay of Gdańsk i) the number of Pseudocalanus sp. populations will decline, ii) the development of individuals representing this species will be inhibited, iii) the total development time may last for more than one year and iv) the first population will be observed at a later date. Numerical analysis shows that the species will develop only in the open sea, mainly in the Southern Baltic, far from the coastal zone and in deeper layers of the studied basin, below the halocline.

Based on the obtained results, it was possible to determine certain forecasts of changes in the population of selected Copepoda species. If climate changes will follow according to scenarios A1B and B1, some major changes in the distribution of the studied organisms are likely to occur. It has been shown that the population of Acartia spp. and Temora longicornis will grow, while the population of Pseudocalanus sp. will become extinct or will reduce its area of distribution. The above-mentioned organisms are the dominant species of Copepoda in the Baltic Sea. The biomass of Pseudocalanus sp. ranges throughout the year from a few to several percent, and from January to March #8211; up to 10% of the total biomass of Copepoda (Dzierzbicka-Głowacka et al. 2010). Reduction in the number of populations of this species or its extinction in some regions of the Southern Baltic, together with the growing number of Acartia spp. and Temora longicornis populations, will significantly affect the quantitative and qualitative structure of Copepoda in the Baltic Sea. At this stage of research it is difficult to foresee the extent of changes in the total biomass of Copepoda. The authors believe that the decline in Pseudocalanus sp. biomass in the total biomass of Copepoda may be offset by an increase in the biomass of Acartia spp. and Temora longicornis. On the other hand, an increase in the abundance of the latter two species, mainly Acartia spp., will considerably exceed the expected decline in the species Pseudocalanus sp.

In the Baltic Sea, zooplankton is the main component of the diet of two fish species caught commercially, i.e. herring (Clupea harrengus) and sprat (Sprattus sprattus). Analysis of the stomach content of these fishes showed that Copepoda are their main food component, and Temora longicornis was the dominant taxon (Casini et al. 2004). Pseudocalanus elongatus minutus dominated in the diet of sprat in winter, while the stomach content of herring (during the same period) was dominated by amphipods, Mysis mixta and polychaetes. Further research conducted by Michelle Casini et al. (2006), covering the period from 1986 to 2004, show a close trophic relationship between herring and the copepod Pseudocalanus elongatus minutus. These studies established a correlation between the absence of this species in the stomach content of herring and the deterioration of the condition of these fishes. One can also find some information in the literature that a decline in the growth of herring, observed in the early 1990s, was mainly caused by a decrease in the amount of food available in the preferred size in the Baltic Proper (Flinkman et al. 1998). Studies conducted by Möllmann et al. (2003; 2004) also show that the deterioration of herring’s condition follows a decrease in the number of Pseudocalanus sp. There are many contradictory reports on the deteriorated condition of planktivorous fish in the Baltic Sea, but most of them predict that a quantitative reduction or removal of Pseudocalanus sp. from the ecosystem may negatively affect the ecosystem functioning and the condition of fish caught commercially.

The conducted analysis can be particularly useful for predicting trophic changes in the seas and oceans. The ongoing changes in the marine environment will directly affect the zooplankton which is one of the main links in the trophic chain, and its availability will affect the associated organisms. Compared to organisms developing in the ocean salinity, those from the Baltic Sea are affected by pauperization. Typical marine organisms, the ontogenetic development of which occurs in the lower salinity, consume greater amounts of energy to maintain the osmotic equilibrium, rather than the growth. Consistent with the forecasts, the future salinity in the Baltic Sea will decrease, which will affect the biomass of a single organism, and consequently the condition of the whole population.