5. Discussion
Examination of the behaviour of this NPZDDF model helps to understand the influencing factors for the dynamics of this system. This work is an attempt to simulate the earlier theoretical work of Ray et al. 2001 applying realistic field data [2]. A detritivorous fish compartment has been added to simulate the detritus compartment more realistically.
This model shows detritivorous fish compete with phytoplankton for available resource indirectly but overall steady state of the system is dependent upon the predation of detritivorous fishes by carnivorous fishes. Detritivorous fishes are even known to enhance the growth of other carnivorous fishes in the mangrove estuarine system [2]. Sundarban mangrove ecosystem is well known for its high fish production which seems to be possible due to the coexistence of detritivorous and other predatory fishes.
Detritivorous fishes in turn are supported by high input of detritus and nutrients from mangrove ecosystem to the adjacent estuary. During monsoon high nutrient availability, both organic and inorganic, brings about dynamic change of food chains [36] by inducing utilization of benthic organic detritus by detritivorous fish and then releasing a portion of the consumed detritus as excretory material which may provide an important source of nutrients to stimulate phytoplankton growth. Rich growth of phytoplankton during postmonsoon due to organic load and washed out materials which are rich in nutrients, leads to subsequent rise in zooplankton population. Assemblage of Z and N in this region induces crowding of carnivorous fishes and other fishes to this region from other parts of estuary. The above findings are in agreement with Ray and Straskraba
[23]. They concluded that this group of fish had no impact on primary production but played a major role in total fish production. Model results show a common parameter (for both DF and CF) uCF, predation rate of carnivorous fish feeding on detritivorous fish is highly sensitive to DF and Z. Slight increase in the value of this parameter causes carnivorous fish biomass to increase as a result zooplankton population is almost become extinct from the system. Similarly, decrease in uCF value causes detritivorous fish biomass to increase significantly. Therefore, it can be said that, uCF plays a critical role in determining the balance not only between two groups of fishes but also population size of P and Z According to Kang and Xiang [36], detritus plays a role of nutrients reservoir, affects the trophic structure and dynamics of communities, and supports a greater diversity of species and longer food chains. Mandal et al. [37] described the PON content of Hooghly-Matla estuarine system during monsoon is the
organic nitrogen content of the sediment soil is about more than 90%. During monsoon period, the mangroves are more prone to tidal influence, which promotes the removal of both organic and inorganic nutrient to the adjacent estuarine water. The model results illustrate that higher biomass of DF are observed during late monsoon period probably due to availability of high food resource. Detritivorous fish are quite efficient in separating organic particles from sediments. D’Avanzo and Valiela [38] commented that nitrogen content is key feature of detrital diets. They found that detritivorous fishes that select detritus with high amino acid content in the form of DON, grows rapidly in contrast to those that do not. Observations of Mandal et al. [37] showed higher DON content during monsoon; these conditions promote rapid fish growth. Yossa and Lima [39] advocated that higher protein content in the detritus during monsoon is caused by allocthonous input. The present finding is consistent with the above statement
Detritivorous fish population declines in very little even when the harvest rates are changed significantly. This indicates that detritus based food chain is more stable and important in comparison to the grazing food in the mangrove estuary and to some extent resistant to anthropogenic perturbations. Fishes of Liza group among the grey mullets are the most suitable for culture in brackish water fisheries because of their intimate association with the mangroves [1]. The other tropical mullets species belonging
to the genera Liza, Mugil and Rhinomugil are also suitable for culture in the mangrove associated areas of Sundarban. The present account deals with impact of mangroves and formation of detritus in the estuarine system.