 |
| Projected sea surface temperature changes by the year 2099 [13]. |
A major component of climate change
is climatic warming, and consequently an increase in oceanic temperatures.
Since water has a high capacity for heat absorption, surface waters tend to
warm while deeper waters retain their cool temperature; this tendency of
surface waters to change in temperature is reflected in the typical measure of
oceanic temperature, Sea Surface Temperature (SST). As SST increases, the
primary production ability of phytoplankton is affected, because the
stratification of the water column that results from dramatic differences in
water temperature limits mixing of critical nutrients from deep cold waters
into warm surface waters. Nutrients such as nitrogen are necessary for
photosynthesis to occur in phytoplankton, as is light availability. These
stratified conditions tend to produce situations in which high light conditions
coincides with the absence of nutrients, and low light is matched with high
nutrient availability, neither of which are acceptable conditions for
photosynthetic growth. Increased SST thus results in low abundances of
phytoplankton, producing a colossal problem for ocean food webs that rely on
phytoplankton primary producers at their foundation.
 |
| The plankton ecosystem, illustrating how phytoplankton depend on nutrient and sunlight availability to fuel subsequent trophic levels, to which they are tightly linked [14]. |
Richardson and Schoeman observed
exactly this pattern in their study of plankton production in planktonic
ecosystems in the Northeast Atlantic. With increases of SST, phytoplankton
production decreased in warmer areas, but increased in cooler areas. The
relationship between these phytoplankton populations and their predators
(copepods in this study, which are herbivorous plankton) was significant, and
supportive of the idea that phytoplankton abundances regulate trophic level
above it in the food chain by bottom-up control. Richardson and Schoeman also
linked their findings to patterns observed in fish ecology due to changing
climate. The changes in phytoplankton abundance may represent the underlying mechanism
of differential cod recruitment in the Northeast Atlantic, where warmer SST in
the south coincides with low recruitment numbers of cod, whereas warming in the
still cool north supports good recruitment. The tight relationship between
plankton trophic levels suggests a high potential for shifts in phytoplankton
production to have large detrimental impacts on fish populations [17]. Coho salmon
have an oceanic life stage during which they are reliant on plankton abundance
to support their diet of small fish. Continued warming of the earth’s oceans
may start to create unfavorable conditions for phytoplankton production in
northern waters where salmon are most prevalent, which would consequently limit
salmon growth, abundance, and reproductive returns due to food scarcity.
 |
| Coho salmon carcasses return nutrients to the stream ecosystem [15]. |
Salmon also have an impact on their
ecosystem, with salmon abundance determining the amount of nutrients that are
recycled back into the environment. Migrating spawners not only provide a
source of food for carnivores such as brown bears and eagles, but the carcasses
of post-spawning salmon are a cache of nutrients that are then infused into the
surrounding stream environment. Upon decomposition, the carcasses release high
quality organic material for use by the stream’s bacteria and algae, determining
steam water nutrient concentrations during late summer and early fall when
spawning occurs and primary production is highest in these streams, according
to Johnston et al. In the study by Johnston et al., salmon carcasses
represented the most importance sources of organic carbon in stream
environments, and their decomposition released pulses of nitrogen and
phosphorus in quantities in excess of immediate environment’s storage capacity.
The large amount of nutrients released by salmon carcasses fuel the local
stream environment, with surplus nutrients exported downstream to larger rivers
and lakes. As a consequence, the abundance of spawning salmon drives primary
productivity in stream environments, therefore supporting the riparian trophic
system [18]. Any climate-related decrease in salmon abundance that results in low
spawner return will then impact the productivity of the stream ecosystem that
so heavily depends on salmon nutrient input.
 |
| Salmon-derived nitrogen is an important soil fertilizer, contributing to the building tree biomass and the increased sequestration of carbon dioxide in terrestrial carbon sinks [16]. |
In addition to stream water
nutrient enrichment, salmon carcass removal by predators has the potential to
transfer significant nutrient material to the terrestrial ecosystem. Johnston
et al. noted that in some coastal watersheds, as much as 10%-35% of
salmon-derived nitrogen and phosphorus may be removed to and invested in terrestrial
vegetation [18]. This nutrient transfer, particularly that of nitrogen, becomes of
great importance with the continued increase in atmospheric carbon dioxide.
Oren et al. studied northern latitude forests for productivity and carbon
sequestration rates and found that carbon transfer from atmosphere to woody
material is limited primarily by nutrient availability. In elevated carbon
dioxide conditions, carbon sequestration is maximized by the addition of
nitrogen fertilizer, so the supplemental nitrogen provided by decomposing
exported salmon carcasses may be key in terrestrial carbon sinks compensating
for increased atmospheric CO2 [19]. Without the presence of
salmon-derived nitrogen, carbon deposition in wood stores may be further
limited, hindering the ability of forests to act as adequate sinks.
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