bioremediation, Capitella, environmental management, fish farm, organic enrichment.

Transcription

1 FISHERIES SCIENCE 28; 74: Bioremediation of organically enriched sediment deposited below fish farms with artificially mass-cultured colonies of a deposit-feeding polychaete Capitella sp. I Kyoko KINOSHITA, 1 Sayaka TAMAKI, 1 Miho YOSHIOKA, 1 Sarawut SRITHONGUTHAI, 2 Tadao KUNIHIRO, 1 Daigo HAMA, 2 Kouichi OHWADA 1 AND Hiroaki TSUTSUMI 1 1 Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, Kumamoto , and 2 Keiten Co. Ltd. Kumamoto , Japan ABSTRACT: For bioremediation of organically enriched sediment deposited below fish farms, the extremely high potential for population growth of a deposit-feeding polychaete, Capitella sp. I, in the organically enriched sediment, and the effect on decomposition of organic matter in the sediment, were examined. A mass-culturing technique was conducted for this species. Bioremediation experiments were conducted on the organically enriched sediment in a fish farm in Kusuura Bay, Japan in Approximately 1.7 million individuals of the worms were placed on the sediment below one net pen in December 23, 9.3 million individuals in November 24, and 2.2 million individuals in November 25. After the worms were spread on the sediment, they rapidly increased in number and reached the highest densities of approximately 134 inds/m 2 in February 24, 527 inds/m 2 in March 25 and 13 inds/m 2 in January 26. In the process of rapid population growth, the decomposition of the organic matter of the sediment was enhanced markedly. Our results demonstrate that the promotion of population growth by spreading cultured colonies of Capitella can enhance the decomposition rate of organic matter markedly in organically enriched sediment below fish farms. This method is promising for minimization of the negative effects of fish farms. KEY WORDS: enrichment. bioremediation, Capitella, environmental management, fish farm, organic INTRODUCTION The development of fish culture has been referred to as the conversion of catching fisheries to rearing fisheries. 1 This new style of coastal fisheries has developed rapidly in various countries throughout the world since the 198s. Fisheries include salmonid culture in western Europe, Scandinavia and North America and a variety of-non-salmonid species (e.g. grouper, sea bream, sea bass, snapper and yellowtail) in Asian Pacific waters. 2 5 Fish farming using net pens is becoming one of the most important ways to obtain seafood resources from coastal areas. 1 Corresponding author: Tel: Fax: kkyoko@pu-kumamoto.ac.jp Received 1 May 27. Accepted 1 August 27. doi:1.1111/j x Many fish farms, however, have caused organic enrichment of the bottom sediments from the vast discharge of organic matter from the net pens. Even at a conservative estimate, 7 66% of the organic carbon contained in the food for cultured fish reaches the sea floor just below the fish farm The organic flux from the net pen to the sea floor ranges between 1.3 and 54.2 g Carbon/m 2 per day (g C/m 2 per day). 12 Although the organic flux changes widely depending on the water depth of the fish farm, it far exceeds that of the natural organic flux, which mainly consists of photosynthetically produced organic matter from phytoplankton and organic debris from terrestrial plants. Accumulation of organic matter on the sea floor below the net pens has a serious negative effect on the benthic environment and on the fish farms because of the dissolved oxygen deficiency in the 28 Japanese Society of Fisheries Science

2 78 FISHERIES SCIENCE K Kinoshita et al. bottom water and generation of high levels of hydrogen sulfide in the organically enriched sediment during the warm seasons. 7,8,13 16 The development of reducing conditions in the bottom environment causes serious disturbance to benthic communities in the enclosed bays where fish farms reside, and the benthic communities become simpler in species diversity, mainly consisting of several species of small polychaetes, and poorer in abundance and biomass. 13,14,17 2 Pearson and Rosenberg 21 and Diaz and Rosenberg 22 predicted such conditions in their schematic models of benthic faunal changes with gradient of organic enrichment of the sediment. Therefore, we need to find effective measures to prevent further worsening of organic enrichment and to re-establish healthy benthic environments for sustainable management of fish farming in enclosed bays. 3,7 In previous studies, several engineering approaches were attempted, including dredging sediment, creation of a sand cover on the sediment, creation of a waterway by digging the sea floor to increase the water exchange and oxygen supply to areas with fish farms, promotion of vertical mixing of the water using a pump 23,24 and location of a sludge collection system under the net pens to minimize the organic loading on the sea floor. 9 However, these approaches have problems in cost, effect and/or efficiency. We have attempted to treat the organically enriched sediment by exploiting the biological activities of a deposit-feeding small polychaete Capitella sp. I. 25 This species and closely related sibling species are common members of the macrobenthic communities in the extremely organically enriched sediment. 21 When the organically enriched sediment is available in laboratory conditions or accessible in the field, Capitella sp. I realize a short life cycle of 4 6 weeks and exhibit very rapid population growth Earlier, we showed that the biological activities of the rapidly increasing Capitella population can be applied to bioremediation of organically enriched sediments in experiments in containers, 31,32 outdoor pools 33 and the innermost areas of an enclosed bay surrounded by heavy industrial areas and a large city. 34 We have conducted bioremediation experiments with a mass culture of Capitella sp. I for treatment of organically enriched sediment deposited on the sea floor below a fish farm in an enclosed bay in Amakusa, Kyushu, western Japan, since In this paper we report the results of bioremediation experiments for the organically enriched sediment in the fish farm from 23 to 25. We discuss the applicability of this technique using Capitella colonies for bioremediation of organically enriched sediment in fish farms in enclosed coastal seas. MATERIALS AND METHODS Study area Kusuura Bay is an extremely enclosed bay located between Amakusa Kamishima Island and Amakusa Shimoshima Island, Kyushu, western Japan (32 23 N, E) (Fig. 1). The water depth at the center of the bay is approximately 16 2 m. In 1973, a fish farm was established in an area of approximately 4 m 2 m at the center of the bay. Approximately 2 t of red sea bream Pagrus major and yellowtail Seriola quinqueradiata have been shipped from the fish farms per year. We set three sampling stations (Stns 1 3) for assessment of the benthic conditions on a transect line from the inside (Stn 1) to the outside of the fish farm (Stns 2 3) in Kusuura Bay from May 23 to October 26 (Fig. 1). Stn 2 was located beside the edge of the fish farm, and Stn 3 was located approximately 4 m from the edge of the fish farm. From May 25 to October 26, we added one sampling station inside the fish farm as a control for Stn 1 (Stn C). Stn 3 Kusuura Bay Amakusa Shimoshima Is. Fig. 1 Amakusa Kamishima Is. Kusuura Bay Stn 2 13 E Stn 1 Kyushu Amakusa Stn C 2 m Fish net pens Study area and sampling site. 33 N 32 N 28 Japanese Society of Fisheries Science

3 Bioremediation with Capitella sp. FISHERIES SCIENCE 79 Sampling procedure We conducted quantitative sampling of the macrobenthic animals and assessment of the chemical conditions of the sediment monthly from May 23 to October 26. At Stn 1, we collected sediments beside the net pens with an Ekman Birge grab sampler (2 cm 2 cm) from a boat, and subsampled ten sediment samples for quantitative sampling of the macrobenthos with a core sampler (5 cm 5cm 5 cm). One sediment sample was collected with an acrylic core sampler (2.9 cm diameter) for chemical analysis of the sediment. From March 24 to October 26 at Stns 1 and C, divers collected three sediment samples for quantitative sampling of the macrobenthic animals and one for chemical analysis with an acrylic core sampler (4.9 cm diameter) at five sites just below the four corners plus the center of the net pen. At Stns 2 and 3, we collected sediments with a grab sampler from the boat, and subsampled ten sediment samples for quantitative sampling of the macrobenthos with the core sampler. We also collected one sediment sample for chemical analysis using a gravity corer with 5-cm long, 4.-cm diameter acrylic pipe from the boat. Analysis of macrobenthic animals For quantitative analysis of the macrobenthic animals, we fixed and stained the sediment samples with 1% formalin solution including rose bengal dye. First, we sieved the sediment samples on a 1-mm mesh screen, sorted the red-stained macrobenthic animals from the residues, and identified and counted them. Next, we sieved the remainder through the 1-mm mesh screen again onto a.125-mm mesh screen, sorted only Capitella species from the residues under a stereoscopic microscope, and counted the number of individuals to determine the population density. Then, we determined the body size (maximum width of the thoracic segments) of the Capitella with a microscope image analyzing system. 35 Finally, we calculated the biomass of the Capitella population as the total wet weight of the population at each sampling station using conversion equations between the body size and dry body weight, and between the dry and wet body weights. 35 Bioremediation experiment with Capitella cultured colonies For the treatment experiment of the organically enriched sediment with mass-cultured colonies of Capitella sp. I, we collected the worms from the sediment just below the fish farm in the study site, and kept the worms in the laboratory as seed colonies for mass culture. We cultured the seed colonies of Capitella in a large tank (1 m 2 cross-sectional area) for two months for field experiments. We set an online vertical profiling system of the water beside the net pen at Stn 1, and monitored daily changes in water conditions. 36 After we had confirmed the recovery of dissolved oxygen level of the bottom water due to the vertical circulation of the water in autumn, we conducted the bioremediation experiment of the organically enriched sediment in the fish farm. We spread approximately 1 68 individuals of cultured worms on the sediment at each of four corners and the center of the area of 12 m 12 m total area just below the net pen (12 m 12 m 8m)atStn1 on 6 December 23, 9 27 individuals on 5 November 24 and individuals on 9 November 25. Organic matter content of sediment We sliced three layers of the sediment from the surface of the gravity core samples every 1 cm depth from June 24 to October 25; the deepest layer was 2 4 cm in depth. The sediment was freeze-dried once, treated with 2 N HCl to remove inorganic carbonate, and vacuum-dried. We determined total organic carbon (TOC) of the sediment with an elemental analyzer (NA-15, Fisons, Rodano-Milan, Italy). Statistical analysis To evaluate the effect of the spread of the Capitella colonies on the organically enriched sediment, we compared TOC of the sediment at Stn 1 in each year from May until before the spread of the colonies (May December 23, May November 24 and May October 25). We also compared TOC of the sediment at Stn C in each year from May until October as a control of Stn 1. The effects of this treatment were tested by a Tukey Kramer test (significance level, P <.5) using StatView v5 software (SAS Institute, Cary, NC, USA). RESULTS Seasonal fluctuations of Capitella population Seasonal fluctuations of the density and biomass of Capitella between May 23 and October 25 are 28 Japanese Society of Fisheries Science

4 8 FISHERIES SCIENCE K Kinoshita et al. shown in Figure 2. From late spring to summer (May September), Capitella were at very low densities in all stations (Fig. 2a). At Stn 1 inside the fish farm, the Capitella population started to recover from October. Then, we spread approximately 1 68 individuals of the cultured worms on the sediment just below a net pen at Stn 1 in December 23, 9 27 individuals in November 24 and individuals in November 25. Approximately one month later, Capitella increased to , and inds/m 2 in 23, 24 and 25, respectively. In the winters (december March) of 24 26, Capitella further rapidly increased and reached the highest density of 1 28 inds/m 2 in January 26. At Stn C, the Capitella population increased during winter, and reached the highest density of inds/m 2 in March 26. At Stn 2 at the edge of the fish farms, the Capitella population also increased in density during the winter; however, population growth was restricted (92 16, and inds/m 2 in February 24, March 25 and January 26, respectively). At Stn 3 outside of the fish farm, the density of Capitella was extremely low throughout the experiments ( 32 inds/m 2 ). The biomass of the Capitella population fluctuated seasonally in the same manner as Capitella densities (Fig. 2b). At Stn 1, the biomass of the population just before spreading the cultured colonies of Capitella was 26., 45.9 and.3 g wet wt/m 2 in December 23, November 24 and October 25, respectively. Biomass increased rapidly and reached 12.4, and g wet wt/m 2 in February 24, March 25 and January 26, respectively. At Stn C, the biomass of Capitella population increased in winter, and reached to 2.1 g wet wt/m 2 in March 26. At Stn 2, the biomass of the Capitella population did not increase markedly, and remained less than 62.7, 14.8 and 71.8 g wet wt/m 2 in the winters of 23 24, and 25 26, respectively. At Stn 3, the biomass of the Capitella population was restricted to less than.2 g wet wt/m 2 during this study. (a) Density (inds/m 2 ) (b) Biomass (g wet wt/m ) 2 MJ JASONDJ FMAMJ JASONDJ FMAMJ JASONDJ FMAMJ JASO Spread cultured colonies of Capitella Spread cultured colonies of Capitella Fig. 2 Seasonal fluctuations in (a) density and (b) biomass of Capitella sp.iatstn1( ), Stn C ( ), Stn 2 ( ) and Stn 3 ( ). Seasonal fluctuations of macrobenthic communities Seasonal fluctuations in the abundance of the macrobenthic communities are shown in Figure 3. All the density data were obtained by sieving the quantitative sediment samples with a 1-mm mesh screen. At Stn 1, the abundance of the macrobenthic communities repeated cyclic seasonal fluctuations characterized by collapse of the communities from the late spring to early summer and rapid recovery from autumn to winter. The macrobenthic communities of Stn 1 mainly consisted of three species of polychaetes Capitella sp. I (Capitellidae), Neanthes caudata (Nereidae) and Prionospio pulchra (Spionidae), and two species of crustaceans Nebalia japanensis (Neballidae, Leptostraca) and Melita sp. (Melitidae, Amphipoda). These five species occupied 88.1% of the total numbers of the macrobenthic animals. Capitella sp. I reached the highest densities of 4 24, and inds/m 2 in February 24, March 25 and January 26, respectively. Nebalia japanensis increased from April, and reached the highest densities of , and inds/m 2 in April 24, May 25 and June 26. Melita sp. showed highest densities of inds/m 2 in May 24 and inds/m 2 in June 26. Macrobenthos at Stn C mainly consisted of three species of polychaetes Capitella sp. I, P. pulchra and Scoletoma longiforlia (Lumbrinereidae) and two species of crustaceans, N. japanensis and Melita sp. These five species occupied 8.% of the total numbers 28 Japanese Society of Fisheries Science

5 Density (inds/m 2 ) Density (inds/m 2 ) Density (inds/m 2 ) Density (inds/m 2 ) Bioremediation with Capitella sp. FISHERIES SCIENCE 81 (a) Stn 1 Capitella sp. I (P) Nebalia japanensis (C) Melita sp. (C) Neanthes caudata (P) Prionospio pulchra (P) Other polychaetes Others (b) Stn C Capitella sp. I (P) Prionospio pulchra (P) Scoletoma longifolia (P) Melita sp. (C) Nebalia japanensis (C) Other polychaetes Others (c) (d) No data Stn 2 Capitella sp. I (P) Prionospio pulchra (P) Theora fragilis (B) Scoletoma longifolia (P) Other polychaetes Others Stn 3 Theora fragilis (B) Chaetozone sp. (P) Tharyx sp. (P) Nephtys sp. (P) Melita sp. (C) Magelona sp. (P) Other polychaetes Others Fig. 3 Seasonal fluctuations in the abundance of the macrobenthic communities at (a) Stn 1, (b) Stn C, (c) Stn 2 and (d) Stn 3. Data are based on sediment samples sieved with a 1-mm mesh screen. P, polychaete, C, crustacean; and B, bivalve. 28 Japanese Society of Fisheries Science

6 82 FISHERIES SCIENCE K Kinoshita et al. of the macrobenthic animals. Capitella sp. I increased from autumn to winter, and reached the highest density of inds/m 2 in January 26. The dominant species of Capitella, P. pulchra, and S. longiforlia were much less than 6 inds/m 2 each. At Stn 2 there were mainly two species of polychaetes Capitella sp. I, and P. pulchra. These dominant species occupied 77.% of the total numbers of the macrobenthic animals. The density of Capitella sp. I was usually less than 5 inds/ m 2 ; however, it increased temporarily to greater than 1 inds/m 2 in February 24 and January 26. The abundance of the macrobenthic animals except Capitella sp. I at Stn 2 was much lower than that at Stn 1 (<3 inds/m 2 ) throughout the year. At Stn 3, the abundance of the macrobenthic communities was also much lower than at Stn 1 (<3 inds/m 2 ). There were more diverse fauna including a bivalve Theora fragilis, an amphipod Melita sp., and various polychaetes including Chaetozone sp., Nephtys sp. and Magelona sp. Seasonal fluctuations of organic matter content of sediment (a) 4 TOC (mg/g dry sediment ) (b) 4 TOC (mg/g dry sediment ) (c) 4 TOC (mg/g dry sediment ) (d) 4 TOC (mg/g dry sediment ) M J J A S ON D J F M AM J J A S O N D J F MA M J J A S O ND J F M A MJ J A S O No data MJ J ASONDJFMAMJ J A S O N D J F MAM J J A S O ND J F MA MJ J A S O MJ J ASONDJFMAMJ J A S O N D J F MAM J J A S O ND J F MA MJ J A S O MJ J ASONDJFMAMJ J A S O N D J F MAM J J A S O ND J F MA MJ J A S O Fig. 4 Seasonal fluctuations in total organic carbon (TOC) of three layers of sediment sectioned from the surface at 1 cm ( ), 1 2 cm ( ) and >2 cm( ) at(a) Stn 1, (b) Stn C, (c) Stn 2 and (d) Stn 3. Data are from June 24 to October 26. The deepest layer was 2 4 cm depth. Seasonal fluctuations in TOC of the three different layers of the sediment at three stations are shown in Figure 4. At Stn 1 where we spread the cultured colonies of Capitella sp. I on the sediment, unique seasonal fluctuation patterns were found in TOC levels of the sediment after the first spread of Capitella in December 23. Before the spread of the Capitella colonies, the TOC levels of the three layers of the sediment ranged mg/g except in July 23, and the differences in TOC among these three layers were within 6.6 mg/g. After the first spread of the Capitella colonies, the TOC levels decreased markedly to mg/g in all three layers of the sediment in January 24, and remained in a low range of mg/g until July 24. These levels in the sediment were even lower than those in the same layers at Stn 2 at the edge of, and Stn 3 outside, the fish farm. After August 24, the TOC levels increased markedly in the top layer ( 1 cm in depth) of the sediment at Stn 1 again, and reached 28.5 mg/g in January 25, while the TOC levels of the deeper layers remained at lower levels. The TOC levels in the second layer (1 2 cm in depth) and the third layer (2 4 cm in depth) of the sediment were 16.8 and 6.7 mg/g, respectively. The difference in TOC between the top and the third layers was 21.8 mg/g. The TOC levels in the top and second layers of the sediment decreased from March 25 when Capitella increased very rapidly from the second spread in November 24 and had established dense patches of more than 3 inds/ m 2. The TOC levels at the top and second layers of the sediment decreased to the same levels as the deepest layer of less than 1 mg/g by May 25. The TOC levels at the top and second layers of the sediment increased from July and August 25, respectively, again as in the summer of 24, but that of the third layer remained at 11.3 mg/g. Afterwards, the TOC levels at the top and second layers of the sediment increased in winter and reached 38.5 mg/g in November 25. However, TOC decreased to mg/g in June. TOC levels in the third layer remained at mg/g. At Stn C, the TOC at the top and second layers of the sediment increased toward winter. TOC levels reached 4. mg/g in the top layer in December 25. After 28 Japanese Society of Fisheries Science

7 Bioremediation with Capitella sp. FISHERIES SCIENCE 83 January 26, the TOC levels in the top and second layers decreased to mg/g in June 26. The TOC levels at the third layer remained at mg/g. At Stns 2 and 3, the TOC of the sediment in all three layers fluctuated in narrow ranges of and mg/g, respectively, during this study. The difference in TOC among the three layers was only less than 3. mg/g at these two stations. We compared the mean TOC levels of the three layers of the sediment at Stn 1 from May until before the spread of the colonies in (May December 23, May November 24, May October 25 and June October 26) to evaluate the effect of the spread of the Capitella colonies on the organically enriched sediment and their rapid population increase in the sediment during winter (Fig. 5). The mean TOC levels in the top layer of the sediment in ranged mg/g, and no statistically significant differences were found among them. In the second and third layers, the decrease in the mean TOC levels of the sediment after the first treatment with Capitella colonies in the winter of was statistically significant (Tukey Kramer test, P <.5). Thus, the effect of the activities of dense patches of Capitella on the sediment appeared in the subsurface layers. Although we did not spread the Capitella colonies at Stn C, Capitella increased from December 25 to April 26. Therefore, we compared the mean TOC levels in the three layers of the sediment at Stn C from May to October between 25 and 26. The TOC levels in the three layers of the sediment ranged and mg/g in 25 and 26, respectively. The TOC level decreased in all layers in 26, and statistically lower values were found in the third layer (Tukey Kramer test, P <.5). Fig. 5 Total organic carbon (TOC) of three layers of sediment in (a) each year from May until before the spread of Capitella colonies at Stn 1 and (b) May October at Stn C., significantly different among years at P <.5 (Tukey Kramer test); error bars, standard deviation (a) Depth -1 cm (n = 6) (n = 6) (n = 5) (n = 5) (n = 5) (n = 5) Depth 1-2 cm Depth > 2 cm (b) 4 Depth -1 cm (n = 6) (n = 6) (n = 5) (n = 5) (n = 5) (n = 5) (n = 6) (n = 5) (n = 5) (n = 5) (n = 5) (n = 5) epth 1-2 cm 4 Depth 2-4 cm 28 Japanese Society of Fisheries Science

8 84 FISHERIES SCIENCE K Kinoshita et al. DISCUSSION The organic flux from the net pen causes organic enrichment of the sediment just below the net pen. 3,6 11 The seasonal changes in the organic flux depend on not only the amount of food spent in the net pen, but also the water column structure. During the warm seasons, the water column tends to be stratified because of low salinity and higher temperature in the surface layer. The organic particles discharged from the net pens disperse widely to outside the fish farm, floating in the surface layer of the low-density regions of the stratified water, while the particles concentrate on the sea floor just below the net pens when the water is vertically well-mixed in autumn and winter. 12 The top layer of TOC in the sediment at Stn 1 therefore increased markedly in September in 24 and 25 (Fig. 4), just after strong winds and waves caused by typhoons had destroyed the water stratification in this study area. In this study, we conducted field experiments to treat the organically enriched sediment deposited just below the fish farm by inducing rapid population growth of Capitella with its artificially cultured colonies in the cold seasons. Figure 6 shows the seasonal fluctuations in the organic flux from the net pen to the sea floor 12 and the decomposition TOC (gc/m /per day) Fig. 6 Seasonal fluctuations in organic flux from the net pen to the sea floor as total organic carbon (TOC, ) and decomposition rate of organic matter by biological activities of Capitella (dtoc, ) at Stn 1. Organic flux data are cited from Tsutsumi et al. 12 The decomposition rate of organic matter was estimated from the biomass data (dry weight, DW) of Capitella in this study and the results of laboratory experiments on the decomposition rate of organic matter of the sediment (dtoc) in a Capitella culture from Chareonpanich et al. 31,32 dtoc (C/m 2 per day) =.15 DW (g) +.23, r 2 =.86; DW (mg) =.2 wet weight (mg) , r 2 =.98. rate of organic matter in the sediment by the biological activities of Capitella at Stn 1. The decomposition rate was estimated from the biomass data of Capitella in this study and the results of the laboratory experiments on the decomposition potential of organic matter in Capitella culture. 31,32 In a laboratory Capitella culture, worms and bacteria decompose the organic matter in the sediment, and the worms stimulate bacterial growth around their burrows. 37,38 Therefore, the estimated decomposition rate of the organic matter in the sediment by the biological activities of Capitella are made up of those of Capitella and bacteria. In the winter of 23 24, the decomposition rate of organic matter in the sediment by the Capitella population reached 2.82 mgc/m 2 per day in February 24, and slightly exceeded or almost balanced the organic flux to the sea floor between January and April 24. In the winter of 24 25, the decomposition rate of organic matter in the sediment by the Capitella population reached 1.12 mgc/m 2 per day by the establishment of dense patches of more than 5 inds/m 2 in March 25, which far exceeded the organic flux to the sea floor in winter. Results of this study demonstrate that the promotion of population growth by spreading the cultured colonies of Capitella can enhance the decomposition rate of organic matter markedly in the organically enriched sediment just below the fish farm. It shows that bioremediation using Capitella is a promising method for minimization of negative effects of fish farms to the benthic environment of the bay by processing the organically enriched precipitated in the fish farm. In this study, we did not spread the Capitella colonies on Stn C; however, Capitella increased at Stn C in winter. These increases were not necessarily reflected in the densities at Stn 1, but the establishment of dense patches may rely on the increase of larval supply from Stn 1. In the benthic communities inside the fish farm, two small crustaceans Nebalia japanensis and Melita sp., and two polychaetes Neanthes caudata and Prionospio pulchra, were dominant in addition to Capitella (Fig. 3). However, they are not infaunal deposit feeders, and do not have large potentials for population growth. It is difficult to expect these species to show large potential for decomposition of organic matter in the sediment. To understand mechanisms why the establishment of dense patches of Capitella colonies exhibit high decomposition potential of organic matter in the organically enriched sediment, we paid attention not only to the feeding activities of subsurface sediments but also the reworking of the sediment. Tube-dwelling infaunal deposit feeders are often known as a powerful sediment bioturbators. They 28 Japanese Society of Fisheries Science

9 Bioremediation with Capitella sp. FISHERIES SCIENCE 85 transport sediment in a vertical direction, bringing sediment from deeper layers to the surface. 39 Capitella is a subsurface deposit feeder, excreting the sediment as fecal pellets and spouting the subsurface sediment on the sediment surface. 4 This reworking potential of the subsurface sediment was estimated from the biomass data of Capitella in the present study and the results of laboratory experiments on burrowing and feeding activities by the worms. 4 We estimated that the densest patches of Capitella with 1 28 inds/m 2 at Stn 1 in January 26 possessed a potential to rework 827 g dry weight sediment/m 2 per day of the subsurface sediment on the sediment surface. It indicates that the Capitella colonies could rework the sediment within less than 1.5 months, assuming that the reworking occurred evenly in the sediment up to 4 cm depth. In this study, we showed that the TOC levels decreased significantly in the subsurface layers of the organically enriched sediment by the spread of cultured colonies of Capitella and their rapid population growth (Fig. 5). The worms feed the subsurface sediment as a subsurface deposit feeder, and decompose organic matter in the sediment. However, the amount of organic matter that the worms consume as a source for secondary production might be limited. 41,42 Madsen et al. 43 reported that in laboratory experiments in microcosms without the worms, there was loss of organic sedimentary contaminants which was confined to the oxidized top layer of the sediment, indicating that microbial processes may have been responsible for some of the loss. These results indicate that organic matter in the sediment is finally decomposed through the microbial community that was enhanced by the biological activities of worms. The worm tubes or burrows create an oxidized layer in the sediment, and provide suitable conditions for an increase in aerobic bacteria in the sediment. 3,37,44 Kunihiro et al. 38 reported a marked increase of aerobic bacteria in organically enriched sediment below a fish farm during winter when the worms established dense patches. Thus, the reworking activities of the sediment by dense patches of Capitella including feeding the subsurface sediment and excreting fecal pellets on the sediment surface, burrowing in the sediment, and spouting the subsurface sediment onto the sediment surface should promote sediment oxidation and provide an oxygen-rich environment suitable for aerobic bacteria in the deeper subsurface sediment. As shown in Figure 5, a large decrease of organic matter content occurred in the subsurface sediment (1 4 cm depth) by the establishment of dense Capitella patches. We are currently investigating methods for the treatment of organically enriched sediment with cultured colonies of Capitella that lead further rapid population growth. We are also developing bacterial agents that associate with Capitella colonies for decomposition of organic matter to realize higher decomposition rates of organic matter in sediments. ACKNOWLEDGMENTS We thank RS Lavin for reviewing the manuscript. This work was supported by a Research and Development Program for New Bio-industry Initiatives of the Bio-oriented Technology Research Advancement Institution (BRAIN) of Japan. REFERENCES 1. Kumai H, Sawada Y. The present conditions and problems of mariculture of fish. In: Kumai O (ed.). Saishin kaisangyo no youshoku. Soubunsha, Tokyo. 2; 2 16 (in Japanese). 2. FAO. Aquaculture production, Fish Circular Rome 1992; 815: Wu RSS. The environmental impact of marine fish culture: towards a sustainable future. Mar. Pollut. Bull. 1995; 31: Ministry of Agriculture, Forestry and Fisheries, Japan. Japanese Fisheries and Aquaculture Production Annual Statistics for Association of Agriculture and Forestry Statistics, Tokyo Ministry of Agriculture, Forestry and Fisheries, Japan. Statistics of Agriculture, Forestry and Fisheries. Ministry of Agriculture, Forestry and Fisheries, Japan. 25. [Cited 7 May 26] Available from URL: www/info/bunrui/bun6.html 6. Tanaka K. Deposition process of pollutants. In: The Japanese Society of Fisheries Science (ed.). Fish Culture in Coastal Areas and Self-pollution. Kouseisha-Kouseikaku, Tokyo. 1977; Gowen RJ, Bradbury NB. The ecological impact of salmonid farming in coastal waters: a review. Oceanogr. Mar. Biol. Ann. Rev. 1987; 25: Gowen RJ, Weston DP, Ervik A. Aquaculture and the benthic environment. A review. In: Cowey CB, Cho CY (eds). Nutritional Strategies and Aquaculture Waste. Fish Nutrition Research Laboratory, University of Guelph, Guelph. 1991; Bergheim A, Aabel JP, Seymour EA. Past and present approaches to aquaculture waste management in Norweigian net pen culture operations. In: Cowey CB, Cho CY (eds). Nutritional Strategies and Aquaculture Waste. Fish Nutrition Research Laboratory, University of Guelph, Guelph. 1991; Holby O, Hall PO. Chemical flux and mass balances in a marine fish cage farm. II Phosphorus. Mar. Ecol. Prog. Ser. 1991; 7: Japanese Society of Fisheries Science

10 86 FISHERIES SCIENCE K Kinoshita et al. 11. Holmer M, Marbá N, Terrados J, Duarte CM, Fortes MD. Impacts of milkfish (Chanos chanos) aquaculture on carbon and nitrogen fluxes in the Bolinao area, Philippines. Mar. Pollut. Bull. 22; 44: Tsutsumi H, Srithonguthai S, Inoue A, Sato A, Hama D. Seasonal fluctuations in the flux of particulate organic matter discharged from net pens for fish farming. Fish. Sci. 26; 72: Tsutsumi H, Kikuchi T. Benthic ecology of a small cove with seasonal oxygen depletion caused by organic pollution. Publ. Amakusa. Mar. Biol. Lab. 1983; 7: Weston DP. Quantitative examination of macrobenthic community changes along an organic enrichment gradient. Mar. Ecol. Prog. Ser. 199; 61: Tsutsumi H, Kikuchi T, Tanaka M, Higashi T, Imasaka K, Miyazaki M. Benthic faunal succession in a cave organically polluted by fish farming. Mar. Pollut. Bull. 1991; 23: Pawar V, Matsuda O, Yamamoto T, Hashimoto T, Rajendran N. Spatial and temporal variations of sediment quality in and around fish cage farms: a case study of aquaculture in the Seto Inland Sea, Japan. Fish. Sci. 21; 67: Brown JR, Gowen RJ, McLusky DS. The effect of salmon farming on the benthos of a Scottish sea loch. J. Exp. Mar. Biol. Ecol. 1987; 19: Tsutsumi H. Environmental impact of fish net pen culture on bottom of a cove, South Japan. Estuar 1995; 18: Tsutsumi H, Inoue T. Benthic environment and macrobenthic communities in a cove with organically enriched bottom sediment due to fish farming for two decades. Benthos Res. 1996; 5: Karakassis I, Tsapakis M, Hatziyanni E, Papadopoulou K-N, Plaiti W. Impact of cage farming of fish on the seabed in three Mediterranean coastal areas. Mar. Sci. 2; 57: Pearson TH, Rosenberg R. Macrobenthic succession in relation to organic enrichment and pollution of the marine environment. Oceanogr. Mar. Biol. Ann. Rev. 1978; 16: Diaz RJ, Rosenberg R. Marine benthic hypoxia: a review of its ecological effects and the behavioural responses of benthic macrofauna. Oceanogr. Mar. Biol. Ann. Rev. 1995; 33: Kimura H. Restoration techniques of polluted fish farms. In: Watanabe T (ed.). Coastal Fish Farms and the Environment. Kouseisha Kouseikaku, Tokyo. 199; Kawai A, Kitada H, Maeda H. Restoration techniques of a small area in polluted fish farms. In: Watanabe T (ed.). Coastal Fish Farms and the Environment. Kouseisha Kouseikaku, Tokyo. 199; Tsutsumi H, Montani S. Utilization of biological activities of capitellid polychaete for treatment of (Hedoro) (organically enriched sediment) deposited on the marine bottom just below net pen culture. Nippon Suisan Gakkaishi 1993; 59: Grassle JF, Grassle JP. Opportunistic life histories and genetic systems in marine benthic polychaetes. J. Mar. Res. 1974; 32: Tsutsumi H. Population dynamics of Capitella capitata (Polychaeta; Capitellidae) in an organically polluted cove. Mar. Ecol. Prog. Ser. 1987; 36: Tsutsumi H. Population persistence of Capitella sp. (Polychaeta; Capitellidae) on a mud flat subject to environmental disturbance by organic enrichment. Mar. Ecol. Prog. Ser. 199; 63: Tsutsumi H, Fukunaga S, Fujita N, Sumida M. Relationship between Capitella sp. and organic enrichment of the sediment. Mar. Ecol. Prog. Ser. 199; 63: Bridges TS, Levin LA, Cabrera D, Plaia G. Effects of sediment amended with sewage, algae, or hydrocarbons on growth and reproduction in two opportunistic polychaetes. J. Exp. Mar. Biol. Ecol. 1994; 177: Chareonpanich C, Montani S, Tsutsumi H. Modification of chemical characteristics of organically enriched sediment by Capitella sp I. Mar. Pollut. Bull. 1993; 26: Chareonpanich C, Tsutsumi H, Montani S. Efficiency of the decomposition of organic matter, loaded on the sediment, as a result of the biological activity of Capitella sp I. Mar. Pollut. Bull. 1994; 28: Tsutsumi H, Montani S, Kobe H. Bioremediation of organic matter loaded on the sediment in outdoor pools with a polychaete, Capitella sp 1. Fish. Sci. 22; 68: Tsutsumi H, Ueda N, Montani S. Environmental recovery project in Dokai Bay, Kitakyushu, Japan: bioremediation of the organically enriched sediments with a polychaete, Capitella sp 1. Proceedings of The Twelfth International Offshore Polar Engineering Conference Kitakyushu, Japan. The International Society of Offshore and Polar Engineers, Mountain View, CA 22; Tsutsumi H, Kinoshita K, Srithongouthai S, Sato A, Nagata S, Inoue A, Yoshioka M, Ohwada K, Hama D. Treatment of the organically enriched sediment below the fish farm with the biological activities of artificially mass-cultured colonies of a small deposit feeding polychaete, Capitella sp I. Benthos Res. 25; 6: Srithongouthai S, Endo A, Inoue A, Kinoshita K, Yoshioka M, Sato A, Iwasaki T, Teshiba I, Nashiki H, Hama D, Tsutsumi H. Control of dissolved oxygen levels of the water in the net pens for fish farming with a microscopic bubble generating system. Fish. Sci. 26; 72: Wada M, Wu SS, Kogure K, Tsutsumi H. Short-term impact of biological activities of a burrowing polychaete, Capitella sp. I, on bacterial abundance and the chemical characteristics in organically enriched sediment. Benthos Res. 25; 6: Kunihiro T, Miyazaki T, Kinoshita K, Satou A, Inoue A, Hama D, Ohwada K, Tsutsumi H. Microbial community dynamics in organically enriched sediment below fish net pen culture with artificially cultured colonies of the polychaete Capitella sp. I. Bull. Soc. Sea. Wat. Sci. Japan 25; 59: Cadée GC. Sediment dynamics by bioturbating organisms. In: Reise K (ed.). Ecological Comparisons of Sedimentary Shores. Ecological Studies, Vol Springer-Verlag Berlin, Heidelberg. 21; Tsutsumi H, Taguchi A, Sakamoto N. Feeding and burrowing behaviors of a deposit-feeding capitellid polychaete, Capitella sp. I. Benthos Res. 25; 6: Forbes VE, Forbes TL, Holmer M. Inducible metabolism of fluoranthene by the opportunistic polychaete Capitella sp. I. Mar. Ecol. Prog. Ser. 1996; 132: Tsutsumi H, Wainright S, Montani S, Saga M, Ichihara S, Kogure K. Exploitation of a chemosynthetic food resource 28 Japanese Society of Fisheries Science

11 Bioremediation with Capitella sp. FISHERIES SCIENCE 87 by the polychaete Capitella sp. I. Mar. Ecol. Prog. Ser. 21; 216: Madsen SD, Forbes TL, Forbes VE. Particle mixing by the polychaete Capitella species 1: coupling fate and effect of a particle-bound organic contaminant (fluoranthene) in a marine sediment. Mar. Ecol. Prog. Ser. 1997; 147: Alongi DM. Microbes, meiofauna, and bacterial productivity on tubes constructed by the polychaete Capitella capitata. Mar. Ecol. Prog. Ser. 1985; 23: Japanese Society of Fisheries Science