[Lo-Feature] September 1999 L&O Featured Articles now posted

[Everett Fee]

The featured article in the September issue of L&O is: 

"Mesozooplankton influences on the microbial food web: Direct and 
indirect trophic interactions in the oligotrophic open ocean" by Albert 
Calbet and Michael R. Landry. It will appear in L&O 44(6): 1370-1380, 
1999. 

This article is freely available at the Web address: 

         http://aslo.org/lo/toc/vol_44/issue_6/1370.pdf

Instructions for reading PDF files are located on the L&O main web 
page:

         http://aslo.org/help/loonline.html

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Introductory comments by Jon Cole (L&O Associate Editor) 

The organisms in a given food web are linked by various interactions 
such as predation or competition. These interactions can be strong or 
weak; direct or indirect. The strong coupling of nonadjacent trophic 
levels can occur via cascading trophic interactions or trophic cascades. 
Cascades are defined as reciprocal predator-prey effects that alter the 
abundance, biomass, or productivity of a population, community, or 
trophic level across more than one link in a food web (Carpenter and 
Kitchell 1993; Hrabcek 1961). Trophic cascades have been extensively 
studied in freshwater environments, where it has been shown that under 
the right conditions the presence or absence of piscivorous fish can 
strongly influence the abundance and size structure of zooplankton, the 
biomass of phytoplankton, and even the net gas exchange with the 
atmosphere (Carpenter et al. 1985; Mazumder et al. 1990; Schindler et 
al. 1997, and many others). While the phenomenon of trophic cascades 
is best established in freshwater lakes, it has also been described in a 
number of other environments, including tropical forests, agriculture 
fields, and northern coastal areas (Pace et al. 1999). But cascades 
have not been well studied in the open ocean. 

Calbet and Landry (1999) examine experimentally the potential for a 
trophic cascade in the oligotrophic open ocean. In this environment the 
dominant phytoplankton fraction is too small (< 2 µM) for efficient dire=
ct 
consumption by mesozooplankton. However, the organic material in 
these small phytoplankton could, in theory, be linked to 
mesozooplankton through intermediate trophic levels. Calbet and 
Landry performed a series of on-deck manipulative experiments to 
examine the nature of these potential indirect linkages.  
Mesozooplankton consumed microzooplankton and nanoheterotrophs, 
which were >5 µM, and this consumption apparently released the 
smaller phytoplankton from predation control. Increasing the density of
mesozooplankton thus enhanced the growth rates of small (< 2 µM) 
phytoplankton and heterotrophs, including bacteria.(possibly due to 
release of DOC). However, scaling these results to the naturally 
occurring density of mesozooplankton in this environment showed that 
these larger consumers would have very little influence on small 
phytoplankton in nature. Manipulations of nanozooplankton in the range 
of their natural density resulted in very strong effects on planktonic 
bacteria, via a cascade of at least two steps. The conclusion is that 
while there may be trophic cascades in the oligotrophic ocean gyres, 
these may not connect the largest and smallest organisms in the way 
that they appear to in freshwater lakes. 

The existence and significance of trophic cascades is controversial 
(Strong 1990). Experiments such as the ones presented here 
demonstrate the potential (or lack thereof) for these cascades under a 
particular set of conditions. As any good paper does, this one opens 
doors to many lines of inquiry. For example, trophic cascades might be 
more significant in less dilute ocean environments, or at other times of 
the year in oligotrophic environments. Perhaps they may be more 
significant in these environments but higher in the food web (i.e., with 
fish included). Based on their incubation experiments, Calbet and 
Landry conclude that detailed studies of the interactions among pico- 
and nanoplankton appear to be the profitable avenue for improving our
understanding of community structure and function in this region. What 
do you think? 

Literature cited 

Calbet, A., and M.R. Landry. Mesozooplankton influences on the 
microbial food web: Direct and indirect trophic interactions in the 
oligotrophic open ocean. Limnol. Oceanogr. 44: 1370-1380. 

Carpenter, S.R., and J.F. Kitchell [eds.] 1993. The trophic cascade in 
lake ecosystems. Cambridge University Press. 

Carpenter, S.R., et al. 1995. Biological control of eutrophication in 
lakes. Environ. Sci. Technol. 29: 784-785. 

Hrbacek, J., M. Dvorakova, V. Korinek, and L. Prochazkova. 1961. 
Demonstration of the effect of the fish stock on the species composition 
of zooplankton and the intensity of metabolism of the whole plankton 
assemblage. Verh. Internat. Verein. Limnol. 14: 192-195. 

Mazumder, A., W.D. Taylor, D.J. McQueeen, and D.R.S. Lean. 1990. 
Effects of fish and plankton on lake temperature and mixing depth. 
Science 247: 312-314. 

Pace, M.L., J.J. Cole, S.R. Carpenter, and J.F. Kitchell. 1999. Trophic 
cascades revealed in diverse environments. Trends in Ecol. Evol. In 
press. 

Schindler, D.E., S.R. Carpenter, J.J. Cole, J.F. Kitchell, and M.L. Pace. 
1997. Influence of food web structure on carbon exchange between 
lakes and the atmosphere. Science 277: 248-251. 

Strong, D. 1992. Are trophic cascades all wet? Differentiation and donor 
control in speciose ecosystems. Ecology. 73:747-754. 

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Everett Fee
Editor-in-Chief, Limnology & Oceanography
343 Lady MacDonald Crescent
Canmore, Alberta  T1W 1H5  CANADA
office: 403/609-2456, fax: 403/609-2400
<lo-editor@aslo.org>
http://www.ucalgary.ca/~efee
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