Effects of energy flux on the distribution of zooplankton in mangrove forests along the coast of Karachi.

Abstract

From Northern Arabian Sea bordering Pakistan this is the pioneer study on the diversity, density and effects of mangrove energy flux on zooplankton community structure. The study focusses on three aspects; Mesozooplankton (MZ) abundance, Copepod diversity and effect of mangrove energy flux on MZ. The study was conducted at four stations (OC, HS, SBM and HBM) covering the inshore waters of Manora channel. Annual and seasonal variations in density of 21 zooplankton group was estimated with respect to change in habitat with different environmental influences. Density of these groups were high at 5m depth. The Copepods were the most abundant meso-zooplankton group at all stations except at OC followed by cladoceran, gelatinous zooplankton, nematodes and polychaete larvae respectively. Cladoceran was high in density (127700 Ind-m-3) at OC. Salinity, temperature, dissolved oxygen and Chl a concentration was found to effect the distribution of MZ. Cladoceran showed strong relation with salinity and Chl a at OC and HBM. Temperature variation effect the distribution of gelatinous zooplankton strong (F=10.22; P=0.008). Highest density of nematode (12133 Ind-m-3) and polychaete larvae (35600 Ind-m-3) were recorded at HBM and HS respectively. Mangrove stations were highly diversified as compare to other two stations. Even though the high abundance was recorded from OC but highest diversity was attributed to the mangrove stations. High abundance and diversity of zooplankton was found during SWM monsoon season. MDS ordination reveals the 80% similarity between SWM and NEM seasons. Highest values of Chl a were obtained in December at mangrove stations HBM and SBM (59μg-L-1 and 72 μg-L-1 respectively). Classical relation of primary producers and zooplankton was recorded at all stations. From September to November the low concentration of phytoplankton biomass was recorded and at the same time the high zooplankton density was noted. Similarity was noted between SBM and HS as SBM receives hydrological influence from HS whereas, OC receives influence from HBM. A total of 69 species of copepods was recorded during this study. 47 species were identified under the order Calanoida, 17 species in order Cyclopoida and 3 species of harpacticoid copepods were identified. Out of 69 species, 23 species has been observed for the first time from Pakistani waters. The family Temoridae, Paracalanidae and Pseudodiaptomidae were the dominant among Calanoid. Within the cyclopoid copepods Corycaeidae, Oithonidae families were dominant throughout this study and 6 species of genus Oithona were recorded xii first time from Pakistan during this study. Euterpinidae family was the most dominant harpacticoid family with the single genus Euterpina acutifrons. Variations in diversity and density was noted between stations. Eucalanus bungii, Eucheata marina and Pontella securifer was totally absent from station HS. Candacia discaudata, Acartia ohatsukai and Copilia vitrea was totally absent at OC. Rhincalanus Sp., Clausocalanus minor, C. karachiensis, C. chierchiae, C. alocki, Candacia sp., Pontella securifer and Calanopia sp. were not recorded at HBM. Effects of mangrove energy flux on zooplankton community was accessed by the phytoplankton production and its trophic partaking to primary consumers which are zooplanktons. Major nutrients concentrations were determined to study the effects of nutrients on primary production. HS, a polluted station, was rich in NH4 and NO3 concentrations as compared to OC. At HBM substantial relation of NH4 was recorded with the phytoplankton biomass. Although nutrients provide energy for the growth of phytoplankton, other physical variables are also responsible for their growth such as temperature, DO and turbidity. Annual findings illustrate that the omnivore zooplankton were the major consumers in mangrove forest among the functional feeding groups of zooplankton. 56% and 59 % space was occupied by the Omnivore group at HBM and SBM respectively. Only 2-4% of the total zooplankton were detritivores. The tidal cycle was covered at HBM to study the mangrove influence. Overall nutrient concentration was high at MT and low at HT indicating outward flux of nutrients from mangroves. The same pattern was noted for phytoplankton biomass. Zooplankton density was high at HT and LTn as during night the zooplankton moves towards the surface. The phenomena of Dial-Vertical Migration was noted during tidal cycle as indicated through the high density of zooplankton during night sampling. The omnivore zooplankton were high during HT and MT. Inverted energy pyramids was formed at LTs where the detritivore zooplankton dominated the other groups. The out-welling of detritus from mangroves might be the possible cause of high abundance of detritivores at LTs. The results reveals that the study area supports high diversity and density of zooplankton. The variations in species composition between stations indicates the effect of environmental influences. The energy flux from mangroves appears to support the high diversity of zooplanktons not only within mangroves but to the surrounding waters outside the Manora Channel.