Zooplankton Community Dynamics Essay

In the present study a positive correlation was noted between total zooplankton and phytoplankton in RPR and SP. Wildly et al. (2007) reported that in freshwater systems total zooplankton abundance may increase with phytoplankton biomass. Whish et al. 2011) also recorded a significant correlation between total zooplankton abundance and phytoplankton biomass and referred that phytoplankton community influences he variations of zooplankton community by way of bottom-up control through food and feeding interactions and species competition. Reduction in phytoplankton density during monsoon at the ponds reduced food source for zooplankton revealing a bottom-up control and were reflected by reduced densities of zooplankton. Dry seasons, winter and pre-monsoon, were associated with entry of comparatively concentrated wastewater at the ponds.

High nutrient content in the wastewater resulted in increase in density of phytoplankton and in turn, increase in zooplankton density. Decrease of zooplankton densities with introduction offish in nods was reported by Park and Shin (2007). Top down control of zooplankton by fish predation was reported by Eng et al. (2009). Several studies had also documented size selective predation of fish upon invertebrate prey (Brooks and Dodson 1965; Dodson 1974; Caret 1980). Fish was reported to consume larger prey as length of the fish get increased (Morris and Mischief 1999).

Traditional pond preparation activities involving liming of pond bottom and release offish eggs were undertaken in NP. These activities induced proliferation of fish spawn in NP during the pre-monsoon. Developing fish spawn at NP fed on the zooplankton, especially rotifer’s, owing to their small mouth sizes. Similar feeding behavior of fish with small mouth sizes feeding on smaller zooplankton was reported by Pillar 1993. Feeding of spawn on zooplankton, particularly the rotifer’s during most part of pre-monsoon reduced the density of the later during that season.

A significant negative correlation (r= -0. 54; p<O. 01) between rotifer densities and fish spawn observed in NP supported predation pressure on the rotifer’s by spawns. In addition to fish predation, high insist copes particularly the Cyclops, which were known to predate upon rotifer’s (Williamson 1983; Stumberger’s and Evans 1984), possibly increased predation pressure on the thriving rotifer community. Percent rotifer densities were thus, minimal in NP (3. 69%), compared to colander and coped percent densities (28. 98% and 65. 99% respectively).

During late pre-monsoon and early monsoon matured spawn were transferred from NP to RPR. This activity reduced predation pressure on zooplankton in NP and their density gradually increased during monsoon to winter… Contrastingly, RPR which received fish fry during late pre- monsoon and early monsoon showed reduced zooplankton densities during monsoon due to predation by fish fry. In RPR, fish sizes were larger (fry and fingerling stages) which in addition to rotifer’s also fed upon the microinstructions. A significant negative correlation of fish with cleansers and total zooplankton were recorded in RPR (Table 7).

Mufti (1990) reported that fry select larger species of zooplankton as their mouth size gets increased. Increase in percent densities of rotifer’s at RPR compared to NP (Figure 2) could be attributed to the relaxation of predation pressure on ten rootlets Owe to presence AT larger sizes TLS Lime to time SP receiver t fingerling from RPR. Increase in fish densities in SP induced greater predation pressure on thriving zooplankton by developing fingerling and yearling. Larger size of fish (fingerling and yearling) in SP fed upon larger zooplankton (coped and cleansers) resulting in reduced densities of the same.

However, smaller zooplankton, particularly rotifer’s, avoided predation (from fish and predatory copes) and increased their population in SP. Significant negative correlation between fish and copes in SP (Table 7) also Justified feeding of larger sized fish on larger zooplankton.. Findings of Akin-Oriole (2003), Hardback (1962), Brooks and Dodson (1965) corroborated with present findings on age specific predation pressures on different zooplankton groups and consequent changes in zooplankton community structures. UP showed presence of bottom-up control but absence of any top -down control.

Absence of fish predation pressure on the zooplankton community allowed only the inter-specific prey predator relationship between the zooplankton to prevail at the pond (Figure 2). The community structure at UP was comparable with that of NP and RPR with few more thriving species than these two ponds. However, the community structure of UP was contrastingly different than that of SP. This could be attributed to the severe predation pressure exerted by the fingerling and yearling at SP changing the community structure completely from microinstructions dominated to equal abundance of microinstructions and rotifer’s.

The zooplankton structure at UP also revealed that the predation pressure of large sized fish on the zooplankton was more severe compared to smaller sized fish. Similar results were obtained by (Ref). According to Krebs (1999), Chanson’s diversity index is more sensitive to rare species whereas Simpson dominance index emphasize on common species. Dammar depend solely on number of species and showed lowest values in NP and highest in SP which showed the lowest and highest number of species respectively.

H/ is dependent both on number of species and their abundance also showed highest value for SP and lowest for NP. Higher diversity, evenness and lower dominance values in SP (Figure 3) were primarily due to presence of many rare zooplankton species especially rotifer’s which were known to thrive in polluted environments and probably survived the predation pressure owing to their small size. High rotifer density reported from atrophic lakes (Candace 1984) corroborate with the present findings.

Diversity indices between NP and RPR were not significantly different while both the ponds showed significant variations in diversity indices when compared to SP. NP and RPR were fed with wastewater for similar duration (twice a week) of inflow while at SP the intake of wastewater was much greater (everyday for 10 hours). Although, development stages of the fish reared at NP and RPR were different, zooplankton community structure at these two ponds was comparable (Figure 2) and much efferent from the zooplankton community structure at SP.

This condition was also reflected by cluster analysis (Figure 7) where NP and RPR were placed in one cluster owing to their similar nature of species composition and abundance. While SP was placed in a separate cluster at a closer cluster distance to NP-RPR. UP having no fish population was placed in a separate much distant cluster. The results suggested that temporal differences in densities of different zooplankton groups at the ponds were influenced by traditional picturesque method by way of top-down control exerted Day TLS AT Deterrent sale-class Ana Diatom-up control exert phytoplankton.