[L&O Featured Article] L&O Featured Article, Vol. 50 (3) May 2005

[lo-feature at aslo.org]

The Featured Article in the May 2005 issue of L&O is:

Bergström, Ann-Kristin, Peter Blomqvist, and Mats Jansson. 2005. Effects of
atmospheric nitrogen deposition on nutrient limitation and phytoplankton
biomass in unproductive Swedish lakes. Limnol. Oceanogr. 50(3): 987-994.

The article is freely available at:

          http://aslo.org/lo/toc/vol_50/issue_3/0987.pdf

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Introductory Comments by R. E. Hecky (L&O Associate Editor)

It is broadly accepted that the chemistry of the atmosphere is changing at
local and global scales.  It is also fact that this changing atmospheric
chemistry is reflected in wet and dry deposition onto watersheds and lakes.
Acidification of lakes by SOx and NOx was the sentinel impact that brought
this atmosphere-lake connection home to aquatic scientists, and that was
quickly followed by the realization that the atmosphere was a primary vector
for persistent organic pollutants (POPs), e.g., DDT, PCB’s, etc., and even
toxic metals, e.g., Hg and Pb, into lakes far removed from the production,
application, and mobilization of these contaminants.  The adverse effects on
aquatic biota from acidic deposition and the potentially toxic effects on
human consumers of POP’s and metals have also been established through
concerted efforts of the global community. In contrast, and surprisingly,
given global concerns about eutrophying aquatic ecosystems, the recognition
that the atmosphere has the potential to enrich lakes through nutrient
deposition from the atmosphere has emerged more slowly.

The late Peter Kilham, over 20 years ago, was the first to bring to my
attention that acid rain was a misnomer and that it should be more
accurately called “acidic nutrient rain” because the acidifying components
were in fact plant nutrients.  Peter felt that the fact that they were
nutrients certainly opened the possibility of stimulation of plant growth,
and consequent potential for alkalinity generation, with acidification being
likely only if the loadings exceeded the capacity of the plant communities
to use the acidic nutrients.  He also recognized that there were other
elements (and other nutrients) being deposited onto watersheds and lakes in
addition to the potentially acidifying nutrient elements, and that good
ecosystems scientists should be considering the effects on aquatic systems
of these other elements as well just focusing on the acidic nutrients.  From
this perspective, I believe Peter would have enjoyed this issue’s featured
article by Bergstrom et al. which evaluates the effects of atmospheric
nutrient deposition on Swedish lakes along a gradient from high deposition
of N near the source areas in the industrialized south of Sweden and
extending all the way to remote lakes in the high northern latitudes.  From
extensive networks monitoring both the chemistry of deposition and the
receiving lakes, Bergstrom et al. establish that atmospheric deposition has
increased nitrogen loading to these lakes and watersheds, changing TN:TP
ratios in lake waters and increasing biomass yields per unit of phosphorus,
which is the limiting nutrient in these systems today.  

The “nutrient” aspect of acidic nutrient rain has been recognized in the
Laurentian Great  Lakes, where imperfect historical analysis of nitrate
concentrations show a rising nitrate concentration through the last century
(e.g., Bennett et al. 1986).  But Bennett did not have the benefit of an
extensive atmospheric deposition monitoring network or the benefit of
multiple lakes along a deposition gradient; thus, his conclusions about the
role of the atmosphere in the nitrifying of Lake Superior were necessarily
speculative. In contrast, the conclusions of Bergstrom et al. are
statistically rigorous and therefore more conclusive.  Atmospheric nitrogen
(and phosphorus) deposition has also been implicated in the eutrophication
of Lake Tahoe through long range transport from upwind major urban areas
(Jassby et al. 1994).  Even longer range transport with observed and
potential effects on receiving ecosystems has been suggested for the
oligotrophic areas of the Atlantic Ocean (Jickells et al. 1998 and Garrison
et al. 2003).  The large surface areas of great lakes and oceans reduce
relatively the influence of runoff borne nutrients from the watershed and
increase their vulnerability to changes in atmospheric loading.  Tamatamah
et al. (2005) recently concluded that up to 70% of the phosphorus loading to
the world’s second largest freshwater lake, and recently eutrophied, Lake
Victoria, may originate from atmospheric deposition.  Clearly, in both small
systems, as elegantly shown by Bergstrom et al. in this issue, and in large
aquatic systems including the oceans, more attention must be paid to how
atmospheric loadings can directly affect algal productivity, biomass, and
species composition and what the indirect effects may be on the dependent
food webs.

References:

Bennett, E. B. 1978.  The nitrifying of Lake Superior. Ambio. 15:272-275.

Garrsion, H., E. A. Shinn. W. T. Foreman, D. W. Griffin, C. W. Holmes, C. A.
Kellogg, M. Majewski, L. L. Richardson, K. B. Ritchie and G. W. Smith. 2003.
African and Asian Dust: From Desert Soils to Coral Reefs.Bioscience 53:
469-480.
  
Jassby, A. D., J. E. Reuter, R. P. Axler, C. R. Goldman, and S. H. Hackley.
1994. Atmospheric deposition of nitrogen and phosphorus in the annual
nutrient load of Lake Tahoe (California Nevada). Water Resour. Res. 30:
2207-2216.

Jickells, T. E., S. Dorling, W. G. Deuser, T. M. Church, R. Arimoto and J.
M. Prospero. 1998. Air-borne dust fluxes to a deep water sediment trap in
the Sargasso Sea. Global Biogeochem. Cycles 12: 311-320.

Tamatamah, R. L., H. C. Duthie and R. E. Hecky. 2005. The importance of
atmospheric deposition to the phosphorus loading of Lake Victoria. (East
Africa). Biogeochemistry 73: 1-20.