[L&O Featured Article]Vol. 48, Issue 4, July 2003

[lo-feature@aslo.org]

Current Featured Article

The Featured Article in the July 2003 issue of L&O is:

Vadeboncoeur, Yvonne, Erik Jeppesen, M. Jake Vander Zanden, Hans-
Henrik Schierup, Kirsten Christoffersen, and David M. Lodge. 2003. 
>From Greenland to green lakes: Cultural eutrophication and the loss 
of benthic pathways in lakes. Limnol. Oceanogr. 48(4): 1408-1418.

This paper is freely available at this Web address:

          http://aslo.org/lo/toc/vol_48/issue_4/1408.pdf

Instructions for reading PDF files are located on the ASLO web page: 

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

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

Introductory Comments by Bob Hecky

This issue's featured article convincingly demonstrates that 
eutrophication incurs losses as well as gains in lake production 
processes.  For too long, eutrophication has been viewed exclusively 
from a pelagic perspective; this is reflected in most trophic 
classification systems being based total phosphorus or chlorophyll 
concentrations in the open waters, ignoring the contribution of 
benthic plants to lake production.  Vadeboncoeur et al. clearly 
demonstrate that there is a negative interaction between 
phytoplankton and benthic plants-as phytoplankton production 
increases, benthic production decreases as phytoplankton increasingly 
shade out benthic plants. Across a range of total phosphorus (TP) 
concentrations spanning three orders of magnitude, phytoplankton 
productivity increased nearly proportionally to TP but benthic 
production fell; total primary production (the sum of phytoplankton 
and benthic production) increased, but by less than a factor of 10.

So what? It is logical to expect the gain in total lake production to 
result in positive gains for consumers. But data in this paper show 
that there is a convergence of 13C in benthic grazers and 
phytoplankton grazers as TP increases (i.e., as phytoplankton 
production dominates lake production); this clearly demonstrates that 
the loss of benthic production pathways results in a loss of trophic 
diversity. Data in the paper also show that there is a shift in the 
13C of top benthic invertebrate predators with increasing TP: at low 
TP concentrations predators feed on both pelagic and benthic 
production, but at high TP they feed exclusively on pelagic 
production.  

What the authors cannot address in this study is the efficiency of 
use primary production. The shifts in isotopic signatures only 
demonstrate that there is a loss of trophic diversity, and a possible 
loss of trophic efficiency, with increasing TP. Animal secondary 
production may be less positively affected by increasing TP than 
total lake production even if individual species respond 
proportionally. These are complex issues that continue to perplex 
trophic ecology, but Vadeboncouer et al. show the way forward and 
illustrate how stable isotopes effectively capture some of these 
changes in trophic processes.

Benthic primary production supports much of the fabulous fish 
diversity found in the African Great Lakes (Bootsma et al. 1996). 
Seehausen et al. (1997) showed that eutrophication and its effects on 
light transparency and quality negatively impact this diversity 
through loss of sexual selection among fishes that require high 
transparency to maintain mating barriers. Less well understood is how 
the loss of benthic algal production erodes the trophic base of these 
diverse fishes and leads to simplified food webs in which trophic 
generalists may outcompete trophic specialists with a consequent loss 
of biodiversity and trophic efficiency.  I sincerely hope that the 
authors of this issue's featured article, or others inspired by their 
work, will extend their analyses to tropical lakes and address such 
questions.

Bootsma, H. A., R. E. Hecky, R. H. Hesslein, and G. F. Turner. 1996. 
Food partitioning in a species-rich community as revealed by stable 
isotope analyses.  Ecology 77: 1286-1290.

Seehausen, O., J. J. M. van Alphen, and F. Witte. 1997. Cichlid fish 
diversity threatened by eutrophication that curbs sexual selection. 
Science 277: 1808-1811.


-- 
_________________________
Paul F. Kemp, ASLO Web Editor
Web: http://www.aslo.org/
Email: webeditor@aslo.org
Ph: 631-632-8796