Comparison of stations after-pond construction period

4.2. Comparison of stations after-pond construction period

A three-year observation (2004 to 2006) showed a significantly lower value of macrozoobenthic biomass at the stations 1, 3 and 5 compared to the stations 2, 4 and 6 (Table 2). This detailed evaluation does not support our hypothesis that the established ponds caused the decrease of the biomass of macrozoobenthic. However, based on a comparison of non-pond and pond sites, we could show that biomass of macrozoobenthic at pond sites on average was significantly lower compared to the biomass value at non-pond sites (Table 5). Also all macrozoobenthic species which showed significant differences had lower biomass at the pond sites (Table 5). This supports our hypothesis that biomass of macrozoobenthic declined significantly due to pond establishment.

Mangrove litter fall were believed to be a source of organic matter for macrozoobenthic both in the mangrove area tidal flat nearby (Odum and Heald 1975; Lui et al ., 2002; Bosire et al ., 2004). In a review, however, Lee (1995) questioned that benthic biomass had a positive relationship with the availability of detritus originating from mangrove leaves. This is supported by the results of Bouillon et al ., (2002) and Hsieh et al ., (2002) who demonstrated by means of stable isotopes that most benthic organisms do not feed on mangrove-derived organic matter. This is in line with the conclusion that macrozoobenthic biomass does not depend only on food availability (mangrove litter fall) but also food quality. Mangrove litter contains tannin which is not palatable for most macrozoobenthic species (Alongi and Sesakumar 1992) and Lee (1999) found negative relationship between solube tannin and biomass of macrozoobenthic in sediment. These conclusions affect the credibility of our hypothesis and force us to look into other explanations.

Sept em ber 8 th – 9 th 2015, Facult y of Biot echnology – Universit as At m a Jaya Yogyakart a

Discharge water contains nutrients, organic carbon and suspended solids which might have antibiotics. Options to prevent and reduce those pollutants entering the environment are to provide settling ponds (Jackson et al ., 2003; Gautier et al ., 2001; Lee 1993; Halide et al ., 2003), ponds with vegetation (Sansanayuth et al ., 1996; Halide et al ., 2003) and mangrove forest as a filter (Nielsen et al ., 2003). The latter of these options has been put into practice at Sembilang. However, whenever pond effluent was discharged directly into creeks, by 1 km down-stream and within 1-2 months after discharge ceased, water quality was recovered (Trott and Alongi 2000). Trott et al ., (2004) found that discharge of pond waste carbon (C) and nitrogen (N) during shrimp harvest periods did not cause eutrophication further downstream. Hence, it is unlikely that at Sembilang waste water from shrimp ponds has been responsible for a difference between pond sites and non-pond area.

Hence, we consider the possibility that fishing may have influenced the biomass of the intertidal macrozoobenthic. Fishing at Sembilang aims at shrimp, fish and bivalves. The number of active fishing vessels amounts to about 1970 of which daily 40-60 are active in our tidal flat area (Djamali and Sutomo, 1999). Before 1999, fishermen used gill nets to catch shrimps for export abroad. They hardly harvested shrimps near the intertidal areas (Djamali and Sutomo 1999). After 1999, almost all fishing boats have been equipped with modified mini trawls to catch shrimps (Purwoko, unpubl. obs.). These boats operate at the intertidal areas during high tide. The trawls move over and through the muddy top layer of the sediment, supposedly disturbing the macrozoobenthic habitats. Since shrimp vessels operate at all stations they may have contributed to the general decrease of biomass over time.

Comparing June 2004, 2005 and 2006 (Table 3), a significantly lower average biomass was found at Sembilang peninsula in June 2005. Further, stations 1, 4 and 5 had lower biomass of macrozoobenthic, too. This was probably due to disturbance of the habitat of macrozoobenthic by a fishery for bivalves. Starting in March 2005, fishermen harvested Anadara at the intertidal flats of Sembilang peninsula. At station

2, they collected T. timorensis instead of Anadara . In May 2005, fishermen harvested Anadara at station 4 and in June 2005 they collected Anadara at station 5, after June, they moved to intertidal areas further North (station 6). It is assumed that a month after massive disturbance, biomass value of macrozoobenthic could not have recovered completely, but it could be recovered after 2 months by showing significantly higher biomass of macrozoobenthic at station 3 which was harvested in

March and April 2005. At least three fishing boats operated by shoveling the sediment and collecting Anadara every day at high tide. However, no fishing boats operated on station 1. Kenny and Rees (1994) reported that the benthic community at a dredged site had not fully recovered after 7 months and Lu and Wu (2000) supported by demonstrating that the benthic community fully recovered in less than

15 months. Mesh width of the net of fishermen boat was 2 cm, and they collected Anadara bigger

than 2 cm (Djamali 1999). In addition, a group of on average 20 people manually collected Anadara at intertidal areas at low tide. In 1999/2000 (Djamali and Sutomo 1999) fishermen caught 130 ton wet weight of Anadara which is equivalent to about 3 ton afdw at intertidal and subtidal areas; in 2005 we estimate a catch of 32 ton wet weight (0.8 ton afdw) per month from the stations 3 to 6 in the period March to June (Purwoko, unpubl. obs.). This can be compared to the average biomass of Anadara

Sept em ber 8 th – 9 th 2015, Facult y of Biot echnology – Universit as At m a Jaya Yogyakart a

Further, Tellina remies also showed no significant difference of biomass value from 2004 to 2006. Field observation showed that T. remies migrated in August 2005. We observed about 15 to 20 individual animals m -2 floating at the water surface during high tide moving North. Massive harvesting of Anadara probably was the triggering of T. remies migration. This behaviour might explain why T. remies showed no significant differences over time.

We concluded that converting mangrove to ponds coincided with decreasing biomass of macrozoobenthic from 1996 to 2006, and from 2004 to 2006 the nearby presence of shrimp ponds coincided with low biomass of the macrozoobenthic. However, loss of mangrove organic matter and discharge of shrimp pond water might not be responsible for decreasing biomass of macrozoobenthic. Habitat disturbance by fisheries was probably the most reasonable explanation for declining macrozoobenthic biomass. Further research required to evaluate the impacts of mangrove restoration and mangrove development such as harbors and industrial area, on macrozoobenthic biomass in 2015 and 2016.