Impact of Physical Disturbance on the Community Structure of Estuarine Benthic Meiofauna

INTRODUCTION

Tropical estuaries are the most fertile areas in the world and that point out the necessity of having a proper knowledge regarding their benthic productivity and the environmental parameters influencing their subsistence. Several studies have been made on the abundance, distribution and ecology of meiobenthos in temperate and sub tropical regions in India but only a very little attention has been paid on the experimental study of meiobenthos in relation to environmental factors due to physical disturbance and literature in regard is very less or nil. Except, a very few high-quality works on the ecological impact of physical disturbance from Central Indian Ocean Basin (Ragukumar et al., 2001; Sharma and Nath, 1997) and some in vivo experiments done in terrestrial regions and shallow waters of temperate regions by Begon et al. (1990). Most of the other studies are ranging from effects of large scale trawling (Tuck et al., 1998) and benthic storm events (Posey et al., 1996) to small scale disturbance caused by mobile bio turbulaiting organisms (Thrush et al., 1991). Much of the extensive literature concerned with how marine benthic communities respond to organic enrichment has concentrated on effects of anthropogenic inputs associated with fresh water run off (Beukema, 1991), aquaculture (Ritz et al., 1989) and sewage disposal (Hall et al., 1997).

It is well known that environmental disturbance affect the distribution of organisms in a given ecosystem. When an area occupied by a set of species is disturbed, re-colonization and succession will occur with a new set of species (Coe, 1956; Schratzberger et al., 2000). Meiofauna are considered as the best indicators of such environmental stress because of their small size and short generation time. Further more they are also reported to have two potential roles in marine systems: (i) as a food for higher trophic levels and (ii) as nutrient generators (Coull et al., 1972; Schratzberger et al., 2002). Marine benthic meiofaunal assemblages are subjected to a variety of physical disturbance events and their response to such events has not been studied in the estuaries of India.

In recent years, the shrimp farming is a fast growing industry in the maritime states of India. In the global scenario there is an increasing attention on the ecological interactions and the impacts of same on benthic community structure (Bejarano et al., 2005). This prompted us to take up the present investigation, as a part of the environmental impact assessment studies of aquaculture industry. Hence this study was undertaken to find out the effect of manual disturbance caused by the removal of overlying sediment from the suction sump (15 m²approximate area) of an aquaculture farm situated at the Southern bank of the Vellar estuary, Southeast coast of India.

MATERIALS AND METHODS

Study Area
The study was conducted during July 2004 in the suction sump of an aquaculture farm (Blue star aqua) situated in the Southern bank of Vellar estuary at Lat. 11 °29’ N and Long. 79 °46’ (South India). The farm was defunct for several years, which caused sedimentation in the suction sump and hence the study was undertaken upon the removal of overlying sediments.

Sampling Stations
Total three sampling stations were chosen in the Southern side of the Vellar estuary, two stations haphazardly chosen were parallel to the shore approximately 20 m in front and another behind the suction sump as well as from the suction sump. In addition a control site with almost similar sediment texture was chosen in the Northern bank (Comparatively less affected by aquaculture plants). All the four stations had almost similar depth of 1-1.5 m. After initial sampling Casuarina poles were staked at all stations to minimize the effect of anthropogenic activities like casting nets.

Sediment Sampling
Pre-disturbance sampling was done on July 18th 2006 from all the four stations. Since initial post disturbance sampling immediately after one day at the suction sump showed only a few number of organisms, we forced to postpone 3 more days to do actual post disturbance sampling (July 23rd, 2006). Glass corers of 2.5 cm inner diameter were used for collecting sediments to a 10 cm depth manually by skin diving at 4 stations: 3 in the Southern side and 1 in the Northern side (control) of Vellar estuary. All samples were collected in duplicate and were extruded from corers and sliced 2 cm interval for vertical distribution study. Each part were placed in to jars with 5% buffered formalin and Rose Bengal (0.5 mg L^-1). Sediment samples for the analysis of Total Organic Carbon (TOC), water content, porosity, pH and temperature were collected along with core samples. Total eight core samples were collected during day time before and after 3 days of disturbance.

Estimation of Physico-Chemical Parameters
Water
Physico-chemical parameters of the superlying water for water temperature, salinity, dissolved oxygen content and pH were analyzed in the site itself with the help of a degree Celsius thermometer, hand refractrometer (Atago, Japan), YSI-55 DO meter (Yellow spring, USA) and pocket pH pen (Hanna, Italy) respectively.

Sediment
The sediment texture was analyzed by dry sieving method and the values were plotted in a trigon plot. Sediment temperature and pH were analyzed in the site itself with the aid of a degree Celsius thermometer and soil pH meter. Total organic carbon content was measured by wet oxidation method followed by titration with ammonium ferrous sulfate (El Wakeel and Riley, 1956) and expressed as mg g^-1. Sediment Water Content (WC) was calculated by determining the difference between the wet and dry weights and was expressed as a percentage. Sediment porosity was determined with the following equation:

(wc/1.02)/{[(1-wc)/2.64] + wc/1.02}

Where, wc is (wet sediment weight-dry sediment weight)/wet sediment weight (Danovaro et al., 1999).

Enumeration of Meiofauna
Meiofauna were separated by a set of two sieves, the upper one with mesh size of 500 μm and the lower one with a mesh size of 42 μm. Animals retained in the lower sieve were counted and sorted group wise with the help of a binocular microscope and considered as meiofauna. The sorted samples were kept preserved for further taxonomical studies.

RESULTS

Pre-Disturbance
Meiofauna
In the pre-disturbance samples, the range of abundance was 820-1380 animals/core sample, represented by nematodes, copepods, kinorhynchs, gastrotrichs, foraminiferans, oligochaets, polychaetes, ostracodes and others. In all the samples nematodes were the most abundant group followed by harpacticoid copepods (Fig. 1 ). Vertical distribution of meiofauna decreased from surface up to 10 cm. In the entire pre-disturbance sample numerical abundance was high on the top 0-4 cm section.


Fig. 1:	Percentage composition of different meiofaunal groups in different stations


Fig. 2:	Variation in TOC and water content in different stations

TOC, Sediment Characteristics and Physicochemical Characteristics
The most noticeable feature of this study was homogeneous nature of the study area. This was evident from the homogeneity of sediment characteristics (silty clay) (Fig. 3) and TOC (6.8-8.7 mg g^-1) (Fig. 2) of the samples taken from 4 sampling sites before disturbance, as well as from the physico-chemical parameters in all the pre-and post-disturbance sites (Table 1).

Post-Disturbance
Meiofauna
Samples collected immediately after the disturbance (1 day) from the site P-SS showed a drastic decrease in the number of meiofauna (19/core sample). Vertical distribution was not considered because the entire site (P-SS) was heavily trampled due to the manual removal of overlying sediment. Samples from stations P-AS1, P-AS2 and P-CS did not show any significant variation in numerical abundance (Fig. 4).

TOC Sediment Characteristics and Physico-Chemical Parameters
TOC showed a drastic reduction in P-SS and P2-SS samples (1.2 and 2.1 mg g^-1). So also before and after disturbance, sediment texture of the samples from suction sump had shown difference from silty clay to sandy loam (Fig. 4), in turns water content and porosity.

Table 1:	The physicochemical parameters, TOC and sediment characteristics in four different sites in three different samples

SS-Suction sump, AS1 and AS2-Adjacent site 1 and 2, CS-Control site, PSS, PAS1, PAS2 and PCS First sampling in post disturbed suction sump, adjacent site 1 and 2 and control site, P2SS, P2AS1, P2AS2 and P2CS-Second sampling in post disturbed suction sump, adjacent site 1 and 2 and control site


Fig. 3:	Triagon plot showing the sediment texture in four different predisturbed sites

Table 2:	Relationship between meiofauna numerical abundance and different physicochemical parameters and sediment characters


Fig. 4:	Triagon plot showing the sediment texture in four different postdisturbed sites

Assessment of correlation showed TOC, porosity and water content in the sites, before and after disturbance showed significant relation to the varied meiofaunal abundance, while, except pH none of the physicochemical parameters had relation to the meiofaunal abundance (Table 2).

DISCUSSION

Observations showed an understandable decrease in the meiofaunal density in the suction sump (by 89%) between the pre and post-disturbance phases, this was quiet evident to understand the intensity of disturbance and its impact on meiofaunal abundance. Ragukumar et al. (2001) and Rodrigues et al. (2001) observed a similar fall in abundance of meio and macro fauna (32%) during the environmental impact assessment studies for polymetalic nodule mining. The comparatively less numerical abundance observed during the initial sampling (PSS) in the suction sump to the adjacent sites and control site was perhaps because of the reduced oxygen penetration into the sediments due to high organic enrichment caused by bio-deposition (Hargrave et al., 1993; Mazzola et al., 1999; Nomaki et al., 2005). The slight increase in meiofaunal density, TOC and migration of nematodes towards the upper 0-2 cm section at PAS1 and PAS2 might be due to the immediate response of meiofauna to the bideposition caused by the discharged sediment plume from the suction sump. The absence or reduced number of fecal pellets of different organisms during microscopic examination of P2AS1 and P2AS2 samples emphasis the extent of impact (20 m away from the suction sump). In a similar study on the impact of salmon’s cage Duplisea and Hargrave (1996) pointed out a clear reduction of the meiofaunal biomass but not of their abundance, resulting in a general reduction in the average nematode body weight.

There was an increase in meiofaunal density (from 19 to 83 per core sample) during second post-disturbance sampling (after 3 days). The same finding was reported by Alongi (1990) in a tropical soft bottom benthic system and it was concluded that the resilient nature of meiofauna may allow them to quickly repopulate in the disturbed sediment. A significant increase in the percentage composition of harpacticoid copepods than that of nematodes was observed. The reason might be the influence of mobility of an organism (Copepod and kinorhyncha) and the regime of water current, which helped them for the easy re-colonization.

Even though the reduced TOC and the sandy sediment texture (clay silt to sandy clay) traced in the P2SS sample was very much correlated with the abundance of meiofauna. It is not utterly considerable, since these observations are from a small area, Vellar estuary and current patterns doubtlessly vary in time, space, strength and direction in all estuaries.

This study has demonstrated experimentally a relationship between physical disturbance and organic enrichment to the abundance of meiofauna in a selected site in the Vellar estuary. Based on our preliminary results it appeared that benthic disturbance caused by the removal of sediment may have harm full effect for short term. This may be a general inference for the distribution of meiofauna and subsequent re-colonization in estuarine disturbed areas. Long term monitoring is essential to derive the time-frame required for re-colonization to touches the original values.

ACKNOWLEDGMENT

The authors are thankful to the authorities of Annamalai University for the facilities provided to carry out this work.