THE USE OF MANGROVE STANDS FOR SHRIMP POND WASTE-WATER TREATMENT

Degraded coastal environmental quality due to mangrove forest conversion is strongly suspected as a cause of the decline of shrimp pond productivity in lndonesia. In addition, the heavy inputs in shrimp culture practices have excessively polluted and enriched the coastal water, which in turn stimulates the growth of pathogenic bacteria. In an effort to prevent further negative effects of shrimp culture practices, experiments were carried out to assess the capability of mangrove stands to reduce pollutants (nitrate, phosphate, total organic matter, and total bacteria) contained in shrimp pond waste-water. Three pond sizes, i.e,5x5, 10x10, and 15x15 m2, three replicates each, planted with Rhizophora mucronata were used as waste-watertreatment ponds. The shrimps (PL-45) were stocked into seven ponds of 500 m2 each, at a density of 20 shrimp/m2. Feed was given 3 times with the total amount 10-3o/o of shrimp biomass per day. Water changes in shrimp ponds were carried out every three days, with 30% of the waste-water channeled into mangrove ponds and held for another three days. The wasle-water was replaced with reservoir water. The water and soil in both shrimp and mangrove ponds as well as in control pond, were sampled every three days prior to water exchange. The NO3-N, PO.-P, TOM concentrations and total bacteria populations were not different among mangrove ponds sizes. NOr-N tended to precipitate, while most of the PO.-P tended to dissolve in water. Total organic miatter (TOM) in mangrove ponds and soil fluctuated at a similar level and pattern with that in shrimp ponds. The population of bacteria in both water and soil of mangrove ponds was slightly lower, even though statistically not significant, than that in shrimp ponds. Thus mangrove stands may have potential for reducing the negative effects of waste-water from shrimp ponds on the environment.


INTRODUCTION
Starting from 1980 shrimp culture has developed at a rapid pace, which has brought about a significant increase in cultured shrimp production.Unfortunately, the increase in production was accompanied by dis- tinct conversion of mangrove forest into shrimp ponds.
Excessive inputs in shrimp culture and extinction of mangroves in the area in turn enhance the growth of pathogenic bacteria which leads to shrimp harvest failure.Since 1994, cultured shrimp production has been decreasing every year and in 199g the production was less than three-quarters of the production in 1992.
Although the ability of mangroves to neutralize pollutants is not proven, based on their abilities to absorb and use nutrients resulting from decomposing organic wastes for growth (Massaut,l gg8), it is probable that mangroves could settle and neutralize wastewater, especially organic wastes (Soemodiharjo and Soeroyo, 1992).The mangrove root system which is commonly dense is able to retain pollutant particles and develop sedimentation (Kartawinata et al., 1978) as well as to allow organic matter decomposition (Boyd, 1999).The pores or lenticell in stilt roots, especially in Rhizophora spp., function to exchange gas allowing the mangrove to grow both in anaerobic and aerobic conditions (Notohadiprawiro, 1978; Nontji,  1984; Soemodiharjo & Soeroyo, 1994).Atmawidjaja (1987) observed the effect of municipalsewage on a mangrove community and concluded that the mangrove community is not harmed by mu- nicipal sewage and to some extent could be used as an organic waste dumping site.In addition, oysters (Crassosfrea rhizophora) attached on mangrove roots and mangrove cockles (Geloina coaxan) dwelling in the mangrove ecosystem are excellent biofilters for shrimp ponds (Suharyanto et a\.,1996;Mangampa   ef a/., 1998; Tjaronge ef a/., 1998)   Based on those potentialities, this study aimed at assessing the capability of mangrove stands, domi-

MATERIALSAND METHODS
Two-year old mang rove stand s, mostly Rh izopho ra mucronata, were separated into the only three pond sizes available, i.e. 5x5, 10x10, and 15x15 m2, the sizes available with 3 replicates each and 1.0 m high dykes.The density of mangrove in each pond was not different, nine trees/m2, Two ponds with no mangroves were used as settlement ponds or controls.Each pond was filled with the water flowing from the shrimp ponds; the water was held for three days prior to sampling and then channeled out into the environment.The water and the bottom soil of the ponds were sampled in each pond every 15 days.The soil samples for each pond were pooled from five different samples collected using a soil auger until 10 cm depth.The variables observed were NO.-N, PO.-P, total organic matter (TOM) concentrations, and the bacterial population as well as benthos and plankton.The benthos were sampled using an Eikman's dredge, at five stations in each pond, and a sieve was used to separate the molluscs from debris and other organisms.The plankton was sampled using a plankton net no. 25 to filter 100 L water from each ponds.The data of NO3-N, PO4-P, and TOM concentrations as well as total bacteria population of each pond were descriptively analysed.The data of each pond were computed to obtain the average data for mangrove ponds.
The shrimp ponds consisted of seven compart- ments (Figure 1), 500 m2each, and were stocked with 20 Pt-451m2.The water in the ponds was maintained at90-100 cm depth.A 1-kwh paddlewheelaeratorwas set in each pond.The feed was given at 10% total biomass per day in the first month and reduced to 5% in the second month and 3% total biomass in the third month.The amount of feed given was adjusted based on the estimation on total biomass every 15 days.The water was changed every three days by allowing 30o/o ol the totalvolume to flow out into the mangrove ponds with replacementfrom a reservoir.!1htt Experiment pond arrangement based on treatments The reservou (1, 2,3 in Figure) was a 1,S00 m2 pond planted with sea weed (Gracillaria verrucosa) on the bottom and oyster (Crassosfrea iredalei) at28 individuals/m2 set 10 cm above the bottom.The water in the reservoirwas pumped out from a deep welland held three days before being allowed to flow into the shrimp ponds (Atmomarsono et al., 1 995).Water and soil from both shrimp ponds (the same two ponds only) and reservoirwere also sampled for NO3-N, POo- P, TOM concentrations and total bacteria pdpulatioir every 15 days.Each shrimp pond sample is expected to represent a row'of ponds which is assumed to be homogenous based on the soilquality.
Both NO.-N and PO.-P of water and bottom soil were analysed using spectrophotometer with brucine sulphate and sodium tartrate methods, respectively (Haryadi et al., 1992).TOM in water was analyzed using a permanganate titrimetric method and in soil as loss on ignition (Melville, 1993).Total bacteria numbers were estimated from colony counts an TCBSA (Thiosulfate Citrate Bile Sucrose Agar).The data of each variable were plotted to produce a linear regression, and the slopes of the regression lines among water of all ponds, exceptihe 5x5 m2 ponds, were slightly lower than in the initial concentrations.In the bottom soil, the concentrations of NO"-N were also decreasing and the concentrations in 5x5 m2 ponds were slightly higher than in the rest of the ponds (Figure 2).Statistically, the decreasing rate of NO.-N concentrations as wellas the initial concentrations were not significantly different (P<0.05)among pond sizes.
The concentrations of PO,-P in water decreased in the first 30 days, but then from day 30 started to fluctuate with a tendency of increasing to the end of the experiment.In the bottom soil, PO,-P concentrations started to increase 15 days earlier than in water.However from day 45 the concentrations kept decreasing until the end of the experiment (Figure 3).In the settlement orcontrol ponds and in the shrimp ponds, the behaviour of NO.-N, PO4-P, and TOM concentrations as well as total bacteria population was similar with that in mangrove ponds.In the case of total bacteria in water and soil, the population was not different among ponds except at the last sam- pling (Figure 6).In shrimp ponds, the bacteria population in water kept increasing while in mangrove and control ponds, it dropped on day 75.In the bottom soil, the bacteria population started to increase 15 days earlier than in water.
Figure 7 shows the changes of TOM concentrations in mangrove, shrimp, and control ponds' The concentrations in the water of all ponds increased in the first 60 days and after that distinctly decreased.In the bottom soil, TOM concentration seems to be more stable than in water.
In shrimp ponds, the concentration of NO.-N be- haved as in both mangrove and control ponds.From No differences were observed in the concentrations of PO,-P both in water and soil among pond sizes.Totalorganic matter (TOM) concentration in water fluctuated very much, increasing in the first 45 and 60 days and then decreasing until the experiment terminated.On the other hand, TOM tended to be more stable in the bottom soil (Figure 4).The highest con- centration in water, 100 ppm, was obtained at day 60 in 5x5 m2 ponds, a day after heavy rainfall.On the same day, TOM concentration was the highest in the bottom soil of all ponds.Surprisingly, high TOM contentwas only followed by an increase of total bacteria population in water but not in soil.The bacteria population in water started to increase on day 45 and exceeded 1.5x103 CFU/ mL at day 75 in 5x5 m2 ponds (Figure 5).In soil' the bacteria population started to increase at day 60 and reached 30x103 CFU/mL in 10x10 m2 ponds.the 45th day after the PL were stocked, NO.-N in- creased but then declined in the last 15 days of the experiment.The addition of feed seemed to tempo- rarily affect the concentration of NO3-N in shrimp and controlponds only (Figure 8).In the bottom soil, NO.-N concentrations declined in an almost similar pat- tern in all ponds.In fact, pond soil acts as a buffer which stabilize environmental conditions (Boyd and   Massaut, 1998).
The inputs, more specifically feed, added to the shrimp pond started to affect PO4-P concentration in mangrove ponds water 15 days earlier than in shrimp and control ponds, The highest concentration of POo-P was observed in shrimp pond water at day 45 after the shrimps were stocked (Figure 9).A dense growth IFR Joumal Vol. 7 Noj.2001 of diatomae in shrimp ponds, indicated by watercolour which turned greenish brown, seemed to start absorbing PO.-P by day 45 causing the concentration of PO4-P to decline.In mangrove ponds, PO.-P uptake by mangrove vegetation and the associated organisms (Boyd, 1999; Robertson and Phillips, 1995)   seems to stabilize PO.-P concentration in water 45 days after the shrimps were stocked.
The reduction of NO.-N concentration in water tended to be higher in mangrove ponds than in either control or shrimp ponds (Table 1).Mangrove vegetation, plankton and other organisms associated with the mangrove ecosystem (Table 2) seemed to be the main users of NO.-N.ln shrimp ponds, the addition of feed at 1.5 kg/500 m2 produced less NO"-N than the amount used by phytoplankton (Figure 10).Nitrification seems to occur intensively due to the thorough dissolved oxygen distribution in the water column reaching the pond bottom.The light brown color of pond bottom soil indicated no anaerobic condition in the pond (Boyd ef a1.,1994).
The concentration of PO.-P slightly increased in mangrove, shrimp and control ponds.Based on the r values in a linear regression, the change of PO'-P was more predictablethan NO.-N concentrations.Even though the concentration of PO.-P was high compared to naturalwater (Boyd, 1979), the ratio of NO.-N to PO.-P was below that which would encourage eutrophication due to NO3-N limitation.Bloom of plankton as a result of eutrophication usually occurs at a N-P ratio more than 10 (Ahmad ef a/., 1998).
The addition of inputs in terms of artificial feed increased the concentration of PO4-P in shrimp pond -1-Shrimp -a-Mangrove ---1--Control water up to0.47 mg/l by day 45^ Then, the concentra- tion started to decrease and reached 0.22 mg/l by day 90.The changes in feed input, 10% of total biomass in the first month, 5% in the second month, and 3% in the third month are suspected to be the main cause.In the first month, the shrimp were so small that not all the feed was consumed.Consequently, in the first 45 days, most of the feed decomposed into inorganic compounds such as phosphate, nitrate, and ammonia as well as unionized ammonia which then flowed into both controland mangrove ponds.In the second and third months, the shrimp were better able to take and consume the feed, which reduced the amount of unconsumed feed.Boyd (1979) and   Poemomo (1 988) reported that one of the main sources of phosphate in shrimp ponds is artificial feed.
The changes of TOM in a linear regression were more unpredictable, especially in the control pond'  Oscillatoia, Brachionus, Acaftia, Nitzschia, Am phora, P leuros igm a, S u i rel I ea Air compared to those of PO,-P concentration.lt seems that more organic matter accumulated in control pond than in both mangrove and shrimp ponds.The increase in TOM concentration followed by the increase of bacteria population was more observable in shrimp ponds Water +T ruoa r.Plants than in controland mangrove ponds.Sedimentation in three days without addition of input and also tannin contained in mangrove litterwere suspected to inhibit the growth of bacteria in water (Atmomarsono ef a/.. 1995; Harahap, 1997).
Figure 10.The cycle of nitrogen in ponds (after Boyd, 1991) The behavior of NO.-N, PO4-P, and TOM concentrations and bacteria populations in pond bottom soil was more or less similar with those in water even though the changes were more unpredictable, except for the bacteria population (Table 3).Ahmad (1998)   observed similar patterns of NO.-N and PO.-P changes in mangrove stands for almost two years.In shrimp pond bottom soil, the growth of the bacteria popula- tion was more than twice that in mangrove pond bottom soil.The litter (leaves and branches) of mangrove containing tannin (Soqtarno, 1997) is suspected to inhibit the growth of bacteria populations (Table 4) in both water or pond bottoms.
Based on the findings above, mangrove stands have potential for shrimp ponds waste-water treatment.
Further, they could be used in an ecofriendly recirculating shrimp culture system.Ahmad (1988), reported thatthe maximum area of mangroveforestwhich could be converted into productive shrimp ponds is less than 20o/o ol the total area based on salinity distribution, amplitude of tide, soil quality and texture, as well as land elevation, However, more in-depth study on the organisms associated with the mangrove ecosystem or the active substances contained in mangrove trees should be carried out to assure the optimal use of mangrove stands in shrimp culture.The use of mangroves for shrimp ponds wastewater treatment is promising for reducing the possi- bility of eutrophication, which is usually followed by disease out-breaks caused by organic pollution gen- erated in shrimp ponds.Further in-depth study on the organisms associated with the mangrove ecosystem for shrimp ponds bioremediation is recommended.
nated by Rhizophora mucronata, to neutralize shrimp ponds waste-water.Knowledge about the capability of mangrove stands to neutralize such waste could 1 Researcher at the Research Center of Aquaculture r Researcher at the Research Institute for Coastal Fisheries.Maros help in the designing an eco-friendly or responsible coastal aquaculture.
ponds, allvariables observed fluctuated with a tendency to slightly decrease.By the end of the experiment, NO.-N concentrations in the Figure 2 Figure 4.
concentrations in water (left) and soil (right) of shrimp, mangrove,

Table 3 .
The regression values of various variables in shrimp, mangrove, and control ponds soil