Year-round accumulation of particulate organic matter in the estuarine turbidity maximum: comparative observations in three macrotidal estuaries (Chikugo, Midori, and Kuma Rivers), southwestern Japan

Abstract

The inner part of the Ariake Sea is one of the most productive estuarine systems in Japan. To examine potential food items for estuarine organisms, we conducted monthly observations of the dynamics of particulate organic matter along the macrotidal Chikugo River estuary in 2005 and 2006. In the neighboring macrotidal Midori and Kuma River estuaries, comparative observations were made. High turbidity and strong vertical mixing were observed only at low salinities (\10) in the Chikugo River estuary. In contrast, the Midori and Kuma River estuaries were characterized by less turbid and less mixed waters. Concentrations of particulate organic carbon often exceeded 5 mg l -1 in or close to the estuarine turbidity maximum (ETM) of the Chikugo River estuary. However, such high concentrations were rarely observed in the other two estuaries. The observed differences could be attributable to different hydrodynamic processes related to the different lengths of tidal reaches: 23, 8, and 6 km in the Chikugo, Midori, and Kuma Rivers, respectively. In the Chikugo River estuary, spatiotemporal changes of chlorophyll a suggested that phytoplankton occurred abundantly up-and/or downstream from the ETM especially during the warm season. In contrast, pheophytin (i.e., plant detritus) always accumulated in or close to the ETM. Carbon stable isotope ratios and carbon to nitrogen ratios indicated that the plant detritus was derived from phytoplankton and terrestrial plants. The Chikugo River estuary has a high potential to support the production of estuarine organisms through abundant plant detritus in the well-developed ETM all the year round.

Relationship between nutrients and plankton biomass in the turbidity maximum zone of the Pearl River Estuary

“…Similarly, the peaks of copepods abundance were also reported in the TMZ in the Columbia River estuary (Simenstad et al, 1994) and in the Gironde estuary in France (David et al, 2005). TMZ is an advantageous site for zooplankton growth by providing preys (Suzuki et al, 2012) and/or increasing survival rates of zooplankton since their predators occurred less frequently there (Parrish, 1989;Islam et al, 2006).…”

Temporal and Spatial Variation of Mixing and Movement of Suspended Sediment in the Macrotidal Chikugo River Estuary

“…The Chikugo River estuary and offshore tidal flats are strongly affected by the intrusion of freshwater from the Chikugo River and the high tidal range (5 m) of the bay. A few studies have been undertaken to assess the ETM and ecosystems of the Chikugo River estuary and Ariake Sea Tanaka, 2009, 2013;Suzuki et al, 2008Suzuki et al, , 2012. Because those studies have largely focused on the ecology of the area or specifically on fish or zooplankton, SSC and salinity distributions were measured during high water at spring tide, and it took 4-5 hours to make observations from the uppermost to the lowermost station.…”

Microdistribution and feeding dynamics of Coilia nasus (Engraulidae) larvae and juveniles in relation to the estuarine turbidity maximum of the macrotidal Chikugo River estuary, Ariake Sea, Japan

“…Interrelations among seasonal changes in freshwater discharge and temperature (a), the temporal occurrence of C oilia nasus (b), and schedules of sampling (c) in the C hikugo R iver estuary from F ebruary 2005 to J anuary 2006. Monthly temperature consists of both mean (horizontal line) and range (vertical line) observed in the surface water through the estuary under a monthly sampling program (Suzuki et al ., ). Filled triangles represent applications of each sampling method.…”

“…Larvae and juveniles feed primarily on cladocerans and copepods, gradually extending their distributional area to the sea (Matsui et al, 1987). Although such ecological characteristics suggest a great dependence of C. nasus on the estuary, few studies have addressed the feeding, growth, and survival of C. nasus larvae and juveniles in relation to the unique macrotidal environment, specifically the well-developed ETM (cf., Suzuki et al, 2012a).…”