The effect of long‐term spatiotemporal variations in urbanization‐induced eutrophication on a benthic ecosystem, Osaka Bay, Japan

Abstract

Detailed spatiotemporal patterns of the influence of urbanization-induced eutrophication on a metazoan benthic community in Osaka Bay were determined using sediment cores and fossil ostracode assemblages from the last 200 yr. Results suggest that total abundance of ostracodes increased in the middle part of the bay as a result of the increase of food supply by eutrophication. Conversely, abundance decreased in the inner bay, likely because of bottom-water hypoxia by eutrophication. The variation in species composition among sites within the bay may have decreased because of the effect of eutrophication, i.e., the dominance of species that prefer food-rich environments throughout all sites. These eutrophication-induced changes occurred around 1900 as a result of Japan's industrial revolution and around 1960 as a result of rapid urbanization, depending upon location.Anthropogenic eutrophication and resulting hypoxia in coastal shallow marine environments has been recognized as a significant problem and intensely researched both over time and in many places (e.g., Boesch et al. 2001;Kemp et al. 2005). Furthermore, their effect on organisms is well recognized from both living organisms and commercial catch data on living resources (e.g., Diaz and Rosenberg 1995; Cloern 2001; Grall and Chauvaud 2002 for metazoan benthic community). Although monitoring studies relate how urbanization-induced eutrophication can influence the metazoan benthic community, long-term (the past 200 yr) records of effects of eutrophication and hypoxia on metazoan benthos are unavailable because of the short time periods covered by most monitoring records.Long-term records are available from well-dated sediment cores containing faunal or floral proxies of past ecosystem conditions (Cooper 1995;Karlsen et al. 2000;McGann et al. 2003;Willard et al. 2003). Ostracoda (Crustacea) have been shown to be a useful metazoan group abundantly fossilized in bay sediments, thus allowing robust faunal analysis in sediment cores (Cronin and Vann 2003). Many ostracode species are sensitive to anthropogenic activities (Ruiz et al. 2005). While several studies using fossil ostracodes to investigate long-term anthropogenic effects are available (Alvarez Zarikian et al. 2000;Cronin and Vann 2003;Yasuhara and Yamazaki 2005), the long-term spatiotemporal variation of anthropogenic effect including eutrophication is known only in a few coastal ecosystems (Ruiz et al. 1997;Yasuhara et al. 2003).Osaka Bay is adjacent to Osaka City, one of the largest metropolitan areas in Japan. Eutrophication is a serious environmental concern because of the great amount of urban wastewater flowing into the bay (ANSIOB 1996). In order to better understand the effects of urbanizationinduced eutrophication on a metazoan benthic community, we studied the long-term temporal changes in geographic distribution of ostracode abundance, species diversity, and faunal composition in Osaka Bay. Geographic and environmental settings of Osaka BayOsaka Bay is an elliptical embayment (,60 km ...

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