Principles of terrestrial ecosystem ecology.pdf

Nutrient Availability
The euphotic zones of the ocean are frequently
nutrient poor. In pelagic ecosystems of the
open ocean, photosynthetic cells in the euphotic
zone are spatially separated from the
benthic supply of nutrients. This contrasts with
terrestrial ecosystems in which transport tissues
carry nutrients directly from the soil to photosynthetic
cells in the canopy. The small size of
phytoplankton causes diffusion to be the major
process that moves nutrients to the cell surface,
as described earlier. Production in these pelagic
ecosystems is therefore generally nutrient
limited, and algal uptake maintains low nutrient
concentrations in the water of the euphotic
zone (Fig. 10.6). Some phytoplankton swim
(flagellates or ciliates) or sink (through changes
in buoyancy) to reduce nutrient limitation
by diffusion. Swimming can increase nutrient
uptake in microplankton by 50 to 200%, but
picoplankton cannot swim fast enough to over-
Depth (m)
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North
Pacific
Nitrate N (µg L -1 )
5 10 15 20 25 30
California
current
North Central
Atlantic
Mid-ocean
gyres
Peru current
Upwelling
currents
Figure 10.6. Depth profiles of nitrate and phosphate
in midocean gyres and upwelling zones of the ocean.
(Redrawn with permission from Saunders; Dugdale
1976.)
Oceans 231
come diffusion (Valiela 1995). Only large-celled
algae can sink fast enough to overcome nutrient
limitation by diffusion.
The nature of nutrient limitation in the open
ocean is a complex consequence of element
interactions. The open ocean is a nutritional
desert, remote from sources of nutrient input.
In the open ocean, phosphorus appears to be
the master element that ultimately limits the
productive capacity of the oceans (Tyrrell 1999,
Sigman and Boyle 2000). Its supply to the open
ocean depends on products of rock weathering
that are transported to the ocean in rivers,
deposited as dust from neighboring continents,
or mixed upward from the deep ocean. Whenever
phosphorus availability increases, nitrogen
fixers such as cyanobacteria generally add
nitrogen until phosphorus again limits their
production. The open ocean, however, seldom
builds up the high nitrate concentrations found
in lakes, and phytoplankton production frequently
responds more strongly to nitrogen
than to phosphorus in short-term experiments
(Fig. 10.7) (Valiela 1995, Tyrrell 1999). Ocean
water converges strongly on a relatively constant
N:P ratio of 14 to 16, suggesting that both
nitrogen and phosphorus frequently limit production.
This Redfield ratio reflects the relative
requirement of the two elements by phytoplankton
and most other organisms on Earth.
Nitrogen limitation is widespread in coastal
oceans, perhaps reflecting denitrification that
occurs in anaerobic sediments (Falkowski et al.
1999).
Trace elements—which are cofactors for
nitrogenase, the nitrogen-fixing enzyme, and
which are also required by other phytoplankton—often
limit ocean productivity. In the
subequatorial gyres, the Subarctic Pacific, and
the Southern Ocean surrounding Antarctica,
surface nitrogen and phosphorus concentrations
are relatively high, and about half of the
available nitrogen and phosphorus are mixed to
depth without being used to support primary
production. In these regions, production fails to
respond to addition of these nutrients, leading
to a syndrome known as high-nitrogen, lowchlorophyll
(HNLC) syndrome (Valiela 1995,
Falkowski et al. 1999). Large-scale ironaddition
experiments in these regions have