The distribution patterns and biomass of bivalves in Segoro Tambak estuary, Sedati, Sidoarjo, East Java

Indonesia's marine waters have a variety of flora and fauna species that live and associate therein. This study used an observation method with a descriptive analysis. The results showed that the bivalvia distribution pattern in Segoro Tambak Estuary looked diverse with a distribution index value of <1 which was in the range of 0.3 - 0.4. The highest average bivalve biomass was temporally at 2.02 ton/km2 in March, and the lowest biomass was 0.95 ton/km2 in January. The highest total average density was 6 ind/10m2 in March and the lowest was 3 ind/10m2 in February. Low wind and current speeds in March caused the highest density and biomass. The content of the organic matter was the following: BOD in the range of 33.38 to 53.89 mg/l from the threshold of <20 mg/l and COD in the range of 242.6-643.11 mg/l from the threshold of 80 mg/l. This showed very high values and far exceeded the threshold, but bivalves can still live in these waters because bivalves are aquatic bioindicators. The bivalve life of Segoro Tambak Estuary is also supported by muddy substrate conditions that are suitable for bivalve life.


Introduction
Indonesia's marine waters consist of various species of flora and fauna that live and are associated therein [1,2]. Approximately 80% or about 8.000 species live in the various depths of Indonesia's marine waters, and the rest are in freshwater [3]. Bivalves are a type of mollusk that live in both freshwater and marine water, mostly as a microphagous. Bivalves as an organism are generally found in marine waters, especially in coastal zones or littoral zones that are widely used by most people [1,2]. The bivalve class includes various kinds of shellfish and mussels as a component of the food chain, and they can also be indicators of water quality monitoring [3,4]. Many bivalves have an economic meaning, namely as food sources such as Anadara granosa, Anadara antiquate, Anadara gubernaculum and Anadara inaequivalve [5].
The spread of bivalves in water is determined by the abiotic, biotic and bivalve tolerance of each of the involved environmental factors. The abiotic factors include water chemistry, the type of substrate, food availability and biotic factors such as the life cycle patterns that are associated with bivalve distribution patterns [6]. Distribution patterns can be defined as patterns of distance between individuals within a population of boundary [7]. Random distribution is a rare thing in nature [8]. Similar distributions can occur when competition between individuals is hard enough that there is an interaction of positive antagonism that encourages the sharing of the same space. However, the most common pattern of spread is clustering [7].
An estuary is closed water located in the downstream part of a river that is linked with the sea, and it is very possible that a mixture between the two will occur [9]. Mixing these two water masses can result in physical changes such as sedimentation. These physical changes can have a major influence on the biota that need to adapt to their environment. The estuary area is a habitat for various kinds of benthic animal organisms, one of which is bivalves [1]. Segoro Tambak Estuary is a location used for fishing for shellfish by the local community [10]. The work of shell fishing is done in the traditional way, but the exploitation of these resources tends to override the principles of natural resource sustainability. The pressure related to the preservation of the shell resources at Segoro Tambak Estuary is feared to increase with the advance in the activity of the people in the region.
So far, there has been no specific research conducted that examines the distribution patterns and biomass of the bivalves at Segoro Tambak Estuary. The purpose of this study was to determine the distribution patterns and biomass of the bivalves at Segoro Tambak Estuary, Sedati, Sidoarjo, East Java.

Tools and material
The tools that were used in this research study included a boat, trawl, basket, water samplers, ice cooler box, plastic bag, refractometer, thermometer, Secci Disk, pH pen, paper tags, permanent marker, Ekman Grab, Global Positioning System (GPS), bivalvia samples, seawater, sediment, HNO3 solvent and ice cubes.

Method
This study used an observation method with descriptive analysis, carried out by describing the data collected, without intending to make conclusions that applied to the public.

Bivalve sampling
Bivalve sampling was carried out using a trawl. A trawl is an item of fishing gear made out of semicircular iron with its sides having a dividing net. The maximum number of trawls used in one boat is three [11]. Garments that have been equipped with a rope are lowered to the bottom of the water and pulled by the boat for one minute at a speed of 3-4 m/min at a predetermined point. Later on, the trawl can be lifted out of the water after three minutes. The bivalves that were already clean from the mud were put into a plastic clip that had been labeled and distinguished by repetition. The bivalve samples that have been in a plastic bag were then put into a cooler box that already contained ice cubes. After that, the bivalve samples that had been labeled were transported using a vehicle to the Laboratory of Anatomy and Cultivation in the Faculty of Fisheries and Marine in University Airlangga to observe their biomass.

Sediment sampling
The sediment sample was taken using an Ekman grab device at the same point as the bivalve sampling. According to [12], the Ekman grab is suitable for soft sediment sampling from a boat because it only can be done at each point on the spot and does not need to be repeated. The Ekman grab was sunk to the bottom of the water, and then the ballast iron was pushed into the water so then the Ekman grab could be closed. The closed Ekman grab was then lifted onto the boat to handle the sediment. After that, the sediment sample was put into a cooler box to be taken to the Soil Mechanics

Water sampling
Seawater sampling was taken to obtain data on the water quality. The water sample was taken from the boat using a water sampler at a depth that was 30 cm below the surface of the water and the collected sample was put into a bottle. The water sample was immediately stored in a cool box filled with ice cubes at a temperature of ± 4 o C. Storing the sample at a temperature of ± 4 o C can preserve a sample for up to 1-2 days in storage. The water samples obtained were immediately taken to the Laboratory of Research and Standardization Agency (BARISTAND) in Surabaya to be tested for ammonia, nitrite, nitrate and COD.

Data analysis
The data analysis was carried out descriptively, which is a matter of describing the data that has been obtained. The research data was processed using calculation parameters with the identification data as a reference. The calculation parameters that were used included a distribution index (Id), density index (Q) and a biomass index (B).

Results
Segoro Tambak Estuary is a clam fishing area. It is the meeting place for several river streams that have a more varied mixture of water and organic matter. The dominant bivalves that live in Segoro Tambak Estuary include Anadaragranosa, Anadarainequivalvis, Anadara gubernaculum, Mactra sp. and Paphia undulate, which are on average 3-6 cm long and 4-5 cm wide. The most common bivalve species that can be found is Anadaraina equivalvis and the least common is Mactra sp. The results of the bivalve identification from Segoro Tambak Estuary can be seen in Figure 1. The results of the research at Segoro Tambak Estuary undertaken for three months of observation showed that the bivalvia distribution pattern at Segoro Tambak Estuary looked to be diverse with a distribution index value of <1, which is in the range of 0.3 to 0.4. The bivalve distribution pattern data can be seen in Table 1. The average biomass value of the highest bivalves was temporarily 2.02tons/km 2 in March and the average biomass value of the lowest bivalves was temporarily 0.95tons/km 2 in January. The temporarily average biomass data of the bivalve at Segoro Tambak Estuary can be seen in Figure 2.

Figure 2.The temporary average biomass data for bivalves at Segoro Tambak Estuary
The highest total average density value was 6ind/10m 2 in March and the lowest density value was 3 ind/10m 2 in February. The density average of the bivalves at Segoro Tambak Estuary can be seen in Figure 3. The test results focused on the sediment fraction of Segoro Tambak Estuary examined at the Soil Mechanics Laboratory of Institut Teknologi Sepuluh November Surabaya showed that Segoro Tambak Estuary was dominated by mud. The results of the Segoro River Estuary sediment testing in the January to March period can be seen in Table 2. The results of the water quality measurements were obtained from direct measurements in the field and also from in the laboratory. Data on the water quality of Segoro Tambak Estuary in the January to March period can be seen in Table 3. The data on the oceanographic conditions that included wind speed and current speed were obtained from the Tanjung Perak Maritime Meteorology Station in Surabaya. The data of the wind and current speed of Segoro Tambak Estuary can be seen in Table 4.

Discussion
The biomass and dispersal of the bivalves in the water is determined by the biotic and abiotic environment, and also the bivalve tolerance of each of the environmental factors [6]. These physical changes can have a major influence on the biota when it comes to them adapting to their environment. The input of the water that comes from the river flow in Segoro Tambak Estuary will also affect the life of the biota inside it. Segoro Tambak Estuary has more than one flowing river, so it has a high content of organic matter [13]. Mixing the two water masses can lead to physical changes such as sedimentation [9].
The bivalve distribution pattern in Segoro Tambak Estuary looks similar to the temporary observations with a distribution value of <1. This is supported by the statement of [14] who revealed that the value of the Morisita index (Id <1) showed that the pattern of the distribution of the biota in the waters was identical. The diversity distribution pattern can occur because bivalve larvae are not settled in the water or substrate when the time comes to metamorphose into adulthood. Larvae react to certain physical chemical factors such as what is present if the substrate is not in a good state. If the biota it requires is not permanent, then there is no metamorphosis [15].
The oceanographic conditions also have an effect on the bivalve distribution patterns in water. This is related to the wind and the current speed. The highest density and biomass documented occurred in March. This is because the wind and the current speed in March had the lowest value compared to