Assessment of marine environment by ABC (abundance biomass comparison) analysis—A case study on western offshore area of Arabian Sea

Assessment of the variations in marine ecosystem can be effectively monitored using benthic fauna because pollutants from any source will ultimately end in the seabed. The benthic communities play an important role in the transfer of materials from primary production through detrital pool into higher tropic levels, including commercially exploitable fish. Majority of the benthic fauna are sedentary and sessile in nature and cannot avoid any environmental perturbation, hence are considered sensitive indicator of change in the environment caused by natural and anthropogenic disturbances. Based on the analysis of abundance and biomass of benthic communities, one very important and effective study in analyzing marine environment i.e. ABC (Abundance-Biomass Comparison) has been adopted in this paper.


Introduction
ABC curves have a theoretical background in classical evolutionary theory of r-and k-selection. In undisturbed states, the community is supposed to be dominated by k-selected species (slow-growing, large, late maturing), and the biomass curve lies above the abundance curve. With increasing disturbance, slow-growing species cannot cope, and the system is increasingly dominated by r-selected species (fast-growing, small, opportunistic), and the biomass curve will be below the abundance curve. The difference between the two curves is given by the W-statistic, which represents the area between them. The ABC method takes into account the number of species included in the analysis. [1] [2] An advantage of the method is that an appropriate data set for any area or time should allow the status of the community to be evaluated without the need for a spatial or a temporal control against which to compare the index obtained, because the biomass is compared with the abundance for the same time and place. [1] [2] This paper aims to explore the validity and possibility of this method by considering the abundance and biomass data around the 21 ONGC offshore installations and drilling rigs (R-12A, HRA, NLM, BHS, NA, NQ, ICP, SHP, BLQ, B-193, B192,  TPP-TCPP,

Study area
The study area extends geographically from 18°16'20.32"N to 22°37'15.27"N, 71° 1'40.59"E to 72°30'0.00"E covering oil and gas fields of ONGC in the Western continental shelf. The cruise route (Fig: 1) and coordinates/water depth of the installations are given bellow.

Field sampling
Based on OSPAR Guideline [3] (Fig: 2) and keeping in view the pipeline network in the vicinity of the platforms in addition to sea state and manoeuvrability of the vessel around the installations.
Samples are collected at stations scattered in circles of 250m, 500m, 1000m, 2000m and 4000m surrounding each installation. Reference samples were collected beyond 10 kms from the installations. Sea bottom dwelling organisms or benthos are collected at each stations by lowering a Van Veen Grab having penetration depth of 10 cms and variable surface coverage of 587.5 cm2 ( small grab) and 1035 cm2 ( large grab).

Laboratory analysis
All the macro faunal samples were processed on board after 48 hrs. of collection using 500 micron stainless steel mesh screen in filtered seawater and material retained on sieve mesh were fixed in 5% formalin Rose Bengal. In biological laboratory, all the fauna was stored, identified up to the lower possible level under the Microscope. Biomass (wet weight) was measured by blotting the sample on a blotting paper and weight was taken by direct weighing on balance. The biomass was calculated in g/m2.

Data analysis
For construction of ABC curve, Primer (version 6), a software made for statistical analysis for ecological study, was used. Abundance i.e. mean density of different macro benthic species around each stations of an installation are counted as numbers/ meter2 and the total biomass of different stations are measured as gm. /m2 as listed tables ( Table1 & Table  2). Now, the tables of abundance and biomass of different stations of a particular installation are incorporated in the software (primer V6). After running the software as "cumulative dominance curve" one ABC curve is formed where in y-axis we will get "cumulative dominance %" and in x-axis "species rank" will appear. The difference between the two curves is given by the W-statistic, which represents the area between them.