The dynamics of microbial and herbivorous food webs in a coastal sea with special reference to intermittent nutrient supply from bottom intrusion

Abstract

Seasonal changes in abundance of planktonic microorganisms, together with some physico-chemical variables, were monitored monthly from May 1999 to March 2002 in the surface water of a coastal bay where nutrients are mainly supplied by intermittent intrusions of deeper water ͑bottom intrusion͒. No significant bottom intrusion was detected in 1999 but large or frequent bottom intrusions were found from June to October in 2000, and again from mid-June only to late July in 2001. These results indicate that there is a different nutrient supply every year, and peaks in the abundance of dominant eukaryotic phytoplankton ͑diatoms and dinoflagellates͒ roughly corresponded to the occurrences of bottom intrusions. By contrast, there was a cyclic seasonal pattern of autotrophic picoplankton ͑APP͒ cell density, which reached maxima in August of every year at very similar levels ͑4.0-5.0 ϫ 10 5 cells ml -1 ͒. Thus, the seasonal abundance of APP was apparently independent of the occurrence of bottom intrusions. Seasonal changes in cell densities of heterotrophic bacteria showed similar trends to the APP, and temperature-dependent growth of both was indicated. The present study suggests that the matter cycling in the bay varies as a result of shifts in the dominant food linkages, from a microbial food web to a herbivorous food web, due to intermittent nutrient supplies from bottom intrusions.

Seasonal changes in the planktonic food web off Cape Esan, southwestern Hokkaido, Japan

“…On the other hand, in Japan, many studies about the plankton food web have been reported from coastal waters such as the Ariake Sound (Nakamura & Hirata 2006), Uwa Sea (Nakano et al 2004), Dokai inlet (Uye et al 1998), Seto Inland Sea (Nakamura et al 1994, Uye et al 1996, Ise Bay (Uye et al 2000) and off Usujiri (Shinada et al 2005). A few studies on the seasonal changes in the planktonic food web have been conducted in offshore waters such as in the Kuroshio and adjacent waters (Nakamachi 2003) and in the Oyashio water (Shinada et al 2001).…”