Influence of plankton community structure on the contribution of bacterial production to metazooplankton in a coastal mesocosm

Abstract

To understand energy passing frorn bacterial production to metazooplankton in an aquatic ecosystem, the relative significance of bacterial and photosynthetic production as carbon sources for metazooplankton was determined in a 14 d coastal mesocosm experiment, using frequent in situ batch incubations with dissolved organic and inorganic '-'C tracers. The contnbution ratio (CR) of bacterial to photosynthetic production as rnetazooplankton food resources fluctuated from 6 to 41 O&, according to successional changes in the plankton community structure in the mesocosm. The lowest CR (6%) occurred when nano-sized autotrophs were highly abundant and appendicularians Oikopleura sp., which are capable of ingesting only pico-to nano-sized particles, increased rapidly. After these small autotrophs disappeared, the CR increased to 30-40%, suggesting that the appendicularians were supported largely by bacterial production at that tlme. When the metazooplankton consisted mainly of copepods. the CR reached about 10% despite the relatively low ratio (ca 15%) of net bacterial community production to photosynthesis; also. the copepods would have been able to exploit bactenal production only through intermedianes such as bactenvorous protists. In this penod with copepods dominant, Gymnodinium mikimotoi was the dominant autotroph and might have been a main source of prey for grazers. However, copepods avoid this dinoflagellate, as often reported. Consequently, not much of the autotrophic production of the dinoflagellate might have been ingested by the copepods. We conducted that nano-sized autotrophs and herbivorous protists were likely to be the main suppliers of photosynthetic production for copepods, as nano-sized bactenvores were the suppliers of bactenal production. These findings suggest that bacterial production and the carbon pathway, 1.e. a microbial loop. are not always the terminus of organic carbon flow in food webs, but can be an integral component of the food pool for metazooplankton in the coastal environment.

Efficiency of a phytoplankton‐based and a bacterial‐based food web in a pelagic marine system

“…Earlier studies in both marine and freshwater systems show that the transfer efficiency from both phytoplankton and bacteria to mesozooplankton can be low, 0.1-1% (Ducklow et al 1986;Koshikawa et al 1999;Havens et al 2000). The food web efficiency in our bacteria-based treatment was comparable to these earlier studies (2%), but the efficiency of the phytoplankton-based food web was higher (22%).…”

“…The food web efficiency in our bacteria-based treatment was comparable to these earlier studies (2%), but the efficiency of the phytoplankton-based food web was higher (22%). The low carbon flux from phytoplankton to mesozooplankton observed in earlier studies has been explained by low edibility of dominating algae and the features of the mesozooplankton (Koshikawa et al 1999;Havens et al 2000). Koshikawa et al (1999) found a higher carbon transfer when appendicularians were more abundant than copepods.…”

“…The low carbon flux from phytoplankton to mesozooplankton observed in earlier studies has been explained by low edibility of dominating algae and the features of the mesozooplankton (Koshikawa et al 1999;Havens et al 2000). Koshikawa et al (1999) found a higher carbon transfer when appendicularians were more abundant than copepods. Autotrophic nanoflagellates dominated the phytoplankton assemblage in the NP treatment in our experiment.…”

“…Heterotrophic nanoflagellates or small ciliates may therefore constitute an additional trophic link in oligotrophic systems (Caron et al 1999;Sherr and Sherr 2002). In moderately nutrient-rich systems, the primary productivity may be dominated by nanoplankton, which can be directly consumed by mesozooplankton (Koshikawa et al 1999). In eutrophic systems the lower food web efficiency is explained by increased abundances of inedible or toxic algae (McCauley et al 1999;Sommer et al 2002).…”

“…We have only found a few studies that directly address the question of the relative efficiency of bacterial and phytoplankton pathways (Koshikawa et al 1996;Havens et al 2000). In these studies the transport of carbon from algae and bacteria up through the food web to mesozooplankton was studied by labeling the basal resources (bacteria and phytoplankton) with radioactive dissolved organic carbon (DOC) and dissolved inorganic carbon (Koshikawa et al 1996;Havens et al 2000).…”