Biology Reference
In-Depth Information
patterns of zonation in the intertidal. Much in the way that Connell (
1961
)
concluded that the upper and lower limits of species distributions in the intertidal
may be set by competition and/or predation, Hawkings and Hartnoll (
1985
)
concluded that bands of both dense and sparse macroalgal populations across
intertidal shores of the United Kingdom were set by competitive interactions.
Specifically, when fronds of the brown algae
Fucus
and
Laminaria
were removed,
all other algal species occurring immediately below them were able to colonize
further up in the intertidal zone. Similarly, competition may also set the lower limit
of those species, as they were also able to colonize the lower parts of the shore
following the removal of their competitors.
7.2.4 Competition for Nutrients
Algae require adequate carbon, nitrogen, and phosphorus for growth and survival
(see also Chap.
4
by Gordillo), and competition among seaweeds for them has been
observed in numerous studies. While the majority of studies on the effects of
nutrient limitation have been done for freshwater phytoplankton communities,
and have generally reported strong competition for nutrients, their conclusions
vary depending on the specific species involved and complex synergies associated
with multiple resources (e.g., nitrogen, phosphorus, and silica) that fluctuate inde-
pendently with one another (e.g., Titman
1976
). Unlike freshwater ecosystems,
nitrogen (namely nitrate NO
3
) is the primary limiting nutrient in the ocean, though
phosphorus may occasionally be in limited supply. The primary source of nitrogen
input into most coastal ecosystems is upwelling of cold nutrient-rich water from
below the thermocline, though terrestrial inputs from runoff may also be important
in some locations (Gorman et al.
2009
). Algae take up the nitrogen directly from the
water column via mass transport and thus uptake rates are strongly influenced by
thallus morphology (“functional form,” surface/volume ratios), tissue and seawater
nitrogen content (concentration gradients), and hydrodynamic flow which in turn
regulate boundary layer formation around the algal thallus (Hurd et al.
1997
).
Consequently, during periods of low nutrients and low hydrodynamic flow, algae
can compete intensely for nutrients. For example, Dayton and Tegner (
1984
) report
that along southern California, USA, the dominant current flow is caused by
longshore currents (see also Winant and Bratovich
1981
). While internal waves
may be important in cross-shore delivery of nutrients, the longshore currents are the
primary source of nutrient delivery into the kelp forest. However, drag imposed on
the moving water by the kelp thalli can reduce current speeds within the kelp forest
by as much as 33% resulting in reduced nutrient delivery to the inner parts of the
forest. Further, the nutrients are taken up by those kelps near the periphery of the
forest resulting in further nutrient depletion within the forest. In fact, Jackson and
Winant (
1983
) concluded that nutrient uptake within the Point Loma, CA, USA,
kelp forests can be 23% per hour, resulting in the water being nutrient depleted by
the time it reaches the interior of the kelp forest, though this may be partially
