Benthic and Pelagic Fauna in an Estuary During a Closed Phase Essay

Abstract: : The purpose of studying the biological organisms in the Mpenjati estuary-beach system is to distinguish between organisms and estimate diversity within the estuary found at different locations along the estuary-beach profile in accordance with factors affecting the daily routines of these marine life and other aspects in which these organisms concur with. The organisms found in these waters play an important role in the ecosystems they function in therefore it is imperative to seek a way to keep a close relationship between the estuary-beach systems to prevent breaks in the ecosystems.

The goal of an estuary-beach system is to be able to sustain and diversify the life that exists in these biomes. ‘Organisms that live in beach-estuary systems are phytoplankton, zooplankton and fish (in the water-column) and microphytobenthos, macrofauna and meiofauna which inhabit the sediment. The above mentioned organisms are the main organisms that drive ecosystem functioning in a beach-estuary system’. If the present environments had to disappear, there would be devastating effects not only in the biological sector but also the environmental and economical aspects.

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To avoid catastrophes among ecosystems, tests need to be performed in essence to keep the ecosystems from being harmed. If, for example, diatoms and dinoflagellates were to be removed from the food web due to certain chemical factors many organisms (mollusc larvae, copepods, etc) will not have a food supply therefore resulting in the extinction of many organisms. This leads to disruptions in the ecosystem. The preservation in estuaries is vital for the study of past life and climates, as well as present living organisms and the abiotic factors influencing them.

Introduction: In the Mpenjati beach-estuary system organisms that contribute to a variety of factors and ecosystem functioning include a diverse range of meiofauna, macrofauna, zooplankton, fish and other species that live in either a benthic or pelagic life-cycle. Meiofauna are a complex group of microscopic animals found in waters domineering along beach-estuarine systems. These organisms are benthic- they need a surface substrate to attach themselves onto e. g. sediments. Meiofauna mainly include nematodes, ploychaetes and harpacticoid copepods. Perissinotto & Smit (2010).

These organisms are important as they clean the waters they live in. Zooplankton organisms can be divided into the pelagic (living in the water-column) or the benthic phases depending on the stages of their life cycle. The animals’ life cycle that lie within the plankton are holoplankton and the ones that spend part of their life cycle in the plankton are meroplankton. Thurman (1997). Zooplankton includes copepods, mysids, amphipods as well as fish larvae, crabs and other benthic organisms. Macrofauna are organisms that can be seen with the naked eye.

These organisms are not large enough to be termed megafauna but they are relatively larger than microfauna. Macrofauna marine animals include crabs, prawns, annelids, insects, amphipods and many other coastal-estuarine water dominated marine species. Many of these organisms are human food sources. Fish are abundant in most bodies of water. Humans use fish as a food source. Fish exhibit the greatest diversity than any other class of vertebrates. Fish use estuarine waters for spawning. In estuarine systems one can expect to find mullets, gobiidae and clupeid species.

Materials and Methods: The procedures at the location on the ocean-estuary-south was to seek information on the different locations that were sampled to account for the variations in not only the biotic factors but also the abiotic factors affecting the present closed beach-estuary system. Zooplankton. One sample was obtained using a hyperbenthic sled(40cm mouth diameter, 200µm mesh) pulled at a distance of 27m. The material collected in the cod-end is placed into a bottle, and fixed with 10% formalin. At the lab the sample was sub-divided in two, diluted with 1L of water and rinsed through a sieve.

On Petri dishes, sub-samples were observed under a microscope where organisms were counted and sorted. To calculate the total volume water filtered, the area of the mouth is multiplied by the tow distance. Perissinotto & Smit (2010). The average number of individuals. m-3 are: mysids- 3337. 94m-3; copepods (acartia. natalensis)- 1381. 86m-3; copepods(psuedodiaptomus hessei)- 15756. 12 m-3. Meiofauna. A twin-corer (20mm internal diameter) collects samples and the sediment is pushed up with a plunger. Only the upper 1cm layer is retained in a vial with 10% formaldehyde.

At the lab the sample was rinsed through the sieve and on Petri dishes observed under a microscope. Organisms were sorted and counted. Perissinotto & Smit (2010). The avg. ind. m-2 of the samples is: nematodes-70063. 69m-2; copepods-3184. 71m-2. Macrofauna. The sampling area is 0. 026m-2. Using an Ekman Grab 4 samples were obtained and preserved. At the lab-rinsed, sieved and observed. Perissinotto & Smit (2010). The average ind. m-2 are amphipoda-346. 15m-2; brachyuran-38. 41m-2; cirripedia larvae-38. 46m-2; isopoda- 153. 85m-2; cumacean- 76. 92m-2; penaeidea shrimp- 384. 2m-2; pontogeloides latipes- 230. 77m-2; polychaetes- 76. 92m-2; mole crab-153. 85m-2; copepods-76. 92m-2. Fish. Fish were captured using a seine net (a coarse mesh) by forming a semi circle in the water and pulling it at an angle back to the shore. Perissinotto & Smit (2010). Catch per unit effort for species caught: mullets-1; gobiidae-1; ambassis gymnocepgalus-1; fish fry-6. Results: Fig. 1 (The number of species & individuals for Zooplankton in Mpenjati estuarine waters): Sub sample 1Sub sample 2 Family/Species:No. of Organisms:Family/Species :No. f Organisms: Copepod (Acartia Natalensis)11Copepod (Accatia Natalensis)25 Copepod (Psuedodiaptomus Hessei)11 Copepod (Psuedodiaptomus Hessei)41 Mysids (Gastrosaccus)15Mysids (Gastrosaccus)91 Fig. 2 (A sample comprising 4 subsets of species for Macrofauna in Mpenjati): Sub sample 1Sub sample 2 Family/Species:No. of Organisms:Family/Species:No. of Organisms: Amphipoda 2Cumacean 1 Brachyura (True Crab)1Penaeidea shrimp4 Cirripedia Larvae 1Pontogeloides latipes (Wide-foot beach louse)3 Isopoda3 Sub sample 3Sub sample 4 Family/Species:No. of Organisms:Family/Species:No. f Organisms: Amphipoda3Emerita austroafricana (Mole crab)4 Cumacean1Insect Larva1 Isopoda 1Melita zeylanica (Brack-water amphipod)4 Larvae1Polychaete spp. 1 Penaeidea shrimp6Pontogeloides latipes (Wide-foot beach louse)3 Polychaete spp. 1Pseudodiaptomus Hessei (Calanoid copepod)2 Tanaeid spp1 Fig. 3 (2 subsets of Meiofauna illustrating the number of individuals & species in Mpenjati): Sub sample 1Sub sample 2 Family/Species:No. of Organisms:Family/Species: No. of Organisms: Eggs (nematodes)1Eggs/ Larvae10 Copepods (Harpocticoicia)1Nematoda (Nematodes)17 Larva (copepods) Nematoda (Nematodes)5 Psuedodiaptomus Hessei (zooplankton)4 Fig4. (The number of species & individuals of Fish in Mpenjati estuarine waters): Sample Family/Species:No. of Organisms: Valamugil robustus (robust mullet) 1 Gobiidae1 Ambassis gymnocepgalus1 Fish fry6 For zooplankton the number of individuals in the subsets (refer to fig. 1) above differ to the “20 individuals of Gastrosaccus( mysids), 100 of Pseudodiaptomus hessei and 200 of Acartia natalensis” identified at the Mpenjati estuary in 2003. Kibirige, Perissinotto (2003). hich respectively has increased, decreased and decreased in numbers. Due to certain limitations many fish species could not be captured although 4 species have been identified and counted (see fig. 4). Fig. 3 illustrates the present-day number of individuals in meiofauna. Fig. 2 indicates a variety of macrofauna in Mpenjati estuary. Discussion: The Mpenjati estuary is a temporary opened/closed beach-estuary system. The Mpenjati is presently closed due to the high-energy wave action from the sea- sediments carried in the seawater were transported and deposited along the sand banks forming a bridge.

The large sand bank therefore does not allow the easy flow of water from the sea to the river mouth resulting in an estuary. Mpenjati is regarded as a river-dominated estuary. In the Mpenjati estuary many kinds of species can be found. This is according to the different locations along the estuary- the salinity, the Ph levels, depth, temperature, rainfall, etc. The zooplankton- pelagic animals were found at a depth of ~0. 7m-0. 8m. The samples collected contained a relatively high amount of organisms but with little diversity due to insufficient marine organisms entering the estuary.

But this does not prevent the abundance of organisms due to the stability of the estuary. During a closed phase waves are unable to form in estuaries enabling a stable estuary. Zooplankton is usually found at surface waters due to the abundance of food sources as they feed off photosynthetic plants and other animals. According to Perissinottoa, Nozaisa, Kibirigeb (2002) “the zooplankton biomass was significantly lower during the open phase (0•02_0•01 g m_3), compared to the closed phase (0•13_0•12 g m_3) (t=3•43, P


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