The population dynamics of a sibling species of CapiteUa capitata (Fabricius) in a cove organically polluted by fish farrn~ng were characterized by early recolonizahon m azoic areas, very rapid population growth, and extinction following development of extremely anaerobic conditions during summer. The remarkably high potential for population growth results from the short life cycle and continual reproduction as a population. Previously, the production of planktonic larvae with the ability to disperse over a wide area was considered one of the most important life history characteristics of opportunistic polychaetes in unpredictable habitats. In the present study, however, Capitella sp. exhibited opportunistic population dynamics during benthic recovery after defaunation in heavily polluted areas, despite production of a small number of large eggs with restricted dispersal ability via lecithotrophic larvae. The size cornposibon of the recolonizing population suggested that some of the worms were recruited as mature adults from a very small population maintalned in the immediate vicinity of the sampling station in an environment disturbed by organic pollution. Although the populahon dynamics of Capitella sp. appear to be characteristic of highly opportunisbc organisms, results suggest that populations may be maintained within each habitat by reliance only on a remarkably large potential for population growth. In general, organically polluted areas may not be temporary habitats that Capitella sp. can utilize only during benthic recovery immediately after defaunation, but rather such areas may be their native habitats.

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Using an indirect and integrated approach, we quantified the magnitude and temporal variabiiity of the contribution of macrozoobenthos to the upward flux of inorganic nitrogen and phosphorus on a sandy tidal flat of the Seto inland Sea, Japan. From Apnl1994 to April 1996, we conducted monthly field surveys on the abundance and faunal composition of the macrozoobenthic communities inhabiting the lower part of the intertidal zone. Subsequently, we carried out 24 h day/night laboratory experiments on the nutrient excretion rate by various size-classes of the dominant species. We then obtained the animal nutrient excretion over a 2 yr period, muitiplying the species-specific excretion rates by the actual animal biomass found on the tidal flat. Additionaliy, for ali different seasons, we calcuiated the upward diffusive flux from the sediments from the vertical profiies of nutrient concentrations in the porewater. On the flat, the bivalves Ruditapes phihppinarum (Veneridae) and Musculista senhousia (Myhhdae) were dominant, making up 86 I 5.6 % when the total biomass exceeded 100 g DW (dry weight) m-'. From our laboratory experiments, the mean (day and night) nutrient excretion rates at 20 to 22OC by the 2 bivalve species were 18.9 p n o l NH4+-N g-' DW h-I, 4.8 pnol (NO3-+ NO2-)-N g-' DW h-' and 3.3 pmol P043--P g-' DW h-I. ln addition, NH4+-N excretion by R. phiiippinarum, but not that by M. senhousia, was significantly higher during the day than during the night. This occurred while the food (Thalassiosua sp.) offered in spikes was rapidly taken up irrespective of the concentration (within a field relevant spring-summer range of 10 to 60 pg T' chl a) andin day/night treatments. The release of nitrogen (N) and the release of phosphorus (P) were highly correlated with each other, for both R. philippinarum and M. senhousia, with a N/P ratio of 7.8 ? 3.0 and 9.9 * 3.5, respectively. In the field, the bivalve excretion rates of nutrients were calculated to be up to 35.2 mmol NH4+-N m-' d-I, 8.8 mrnol (NO3-+ NO2-)-N m-' d-' and 5.8 mmol P O~~--P m-' d-'. These values rank in the upper range of nutrient excretion by intertidal macrozoobenthos and are comparable to those found on dense assemblages (800 to 2000 g ash free DW m-') of the most investigated mussel, Mytzius eduiis. In addition, nutrient fluxes through bivalve excretion varied strongly within a few months, up to 10-fold (R. philippinarum) and 100-fold (M. senhousia) between April 1994 and August 1994, as related to the temporal change of bivalve standing stock. The extent of nutrient regeneration through diffusive flux was comparable to that reported for other intertidal zones of Japan and in the eutrophic Seto Inland Sea, within a range of 0.2 to 1.5 mmol NH4+-N m-' d-' and 0.01 to 0.05 mmol P043--P m-' d-I, thus more than 1 order of magnitude lower than that due to the excretory activity by R. phiiippinarum and M. senhousia. Our resuits indicate that the dominant bivalves, R. phihppinarum and M. senhousia, play a major role in the processes of benthic...

Laboratory colonies of Capitella sp., which 1s found densely distributed in organically enriched or polluted areas, were cultured in sediments with various levels of organic matter to examine the relationship between level of sediment organic matter and growth of individuals. Growth was independent of absolute levels of sediment organic matter, but was significantly correlated with increases in the level of sediment organic matter on addition of algal powder to the sediment. These results indicate that Capitella species predominating in organically enriched areas have a physiological requirement for organic materials discharged from sources of organic enrichment, e.g, abundant growth of algae, fish farming, industrial effluent, sewage, if they are to grow normally. Although associat~on of some Capitella species with sediment organic enrichment has been previously recognized as a reflection of their opportunistic characteristics, a physiological requirement for organic materials discharged from sources of organic enrichment may be the single factor most responsible for the concentration of CapiteNa species in organically enriched areas. However, it is unlikely that they can directly ingest and assimilate such organic materials. Since the additional organic materials are easily decomposed in the sediment, Capitella species may require either (1) some specific microorganisms, the levels of which increase with levels of the organic materials, or (2) certain substance(s) produced during decomposition of the organic materials.

The exclusive domination of several sibling species of the polychaete complex Capitella capitata in organically enriched areas has been recognized to reflect their opportunisbc characteristics, especially produchon of planktonic larvae with widespread dispersal ability. In the present study it was found that a population of Capitella sp. in an organically enriched mud-flat habitat subject to environmental disturbance was, by contrast, maintained without reliance on recolonization from other habitats throughout 1 yr. Although the population declined markedly after the development of reducing conditions in the sediment in early summer, very small patches with extremely low densities (< 100 ind. m-') were preserved in restricted areas with moderate organic enrichment within a gradient of organic enrichment. These remnant populations rapidly reconstructed dense patches in the most organically enriched areas during recovery of sediment conditions from late autumn to winter. Although the most enriched areas allowed CapiteUa sp. to establish dense patches, these areas were accessible only during parts of the year. Nevertheless, the distribution was concentrated in enriched areas, since this depositfeeder required organically enriched sediment for normal growth. The population size structure was apparently influenced by the level of sediment organic matter, and Individuals large enough for reproduction were rare in less enriched areas. Thus, the association with organically enriched sediment and the population dynamics, as characterized by the dramatic seasonal fluctuations in population size, are attributable to the physiological requirement of this species for organically enriched sediment, the dramatic seasonal fluctuations in the carrying capacity of habitats subject to catastrophic environmental disturbance, and the extraordinarly large potential of Cap~tella sp. for rapid populat~on growth.

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 2003-2006. Approximately 1.7 million individuals of the worms were placed on the sediment below one net pen in December 2003, 9.3 million individuals in November 2004, and 2.2 million individuals in November 2005. After the worms were spread on the sediment, they rapidly increased in number and reached the highest densities of approximately 134 000 inds/m 2 in February 2004, 527 000 inds/m 2 in March 2005 and 103 000 inds/m 2 in January 2006.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.

Fish farming using net pens has become very widespread in the coastal areas of various countries in the sediment and to prevent further progress of the organic enrichment of the sediment below the fish farm.