Download The relationship between global warming and decomposition rates

The relationship between global warming and decomposition rates in aquatic systems
Introduction
World-wide, the effects of climate change have become a much discussed topic, in particular the
effects of increased warming. The rate at which the Earth is warming is unparalleled compared to
historical times (Parmesan & Yohe, 2003). A time series compiled jointly by the Climatic Research
Unit and the UK Met Office showed that twelve out of thirteen years between 1995-2007 were the
warmest since records began; with 1998 the warmest ever recorded (Brohan et al., 2006). The
Forth Assessment Report (AR4) by the IPCC indicates that, with the lowest emissions scenario,
global surface temperature is likely to escalate by a further 1.1 to 2.9°C over the next century.
Under the highest emissions scenario this increase could be as high as 2.4 to 6.4°C (IPCC, 2007).
Changes in temperature will have effects on both physiology and behaviours of plants and
animals, as well as ecosystem processes (Calderon-Zavala et al, 2004; Jaio et al, 2009; Beveridge
et al, 2010). Energy is the means by which ecological systems and their processes are run. On an
individual level, energy is expressed as the metabolic rate of an organism. Although metabolic rate
operates on the individual basis, it can be scaled up to the level of whole ecosystems. Studies
show that temperature is a determinant of fundamental biological rates (alongside body mass),
and that warming tends to favour the development and prevalence of smaller organisms (Forster,
Hirst & Woodward, 2011; Yvon-Durocher et al., 2011).
One of the main pathways for nutrient cycling occurs by way of decomposition of dead matter.
Decomposition is central to ecosystem functionality and detritivores are the drivers of this process
(Vos, V. C. A. et al, 2011). These heterotrophic organisms obtain nutrients, such as organic
compounds of carbon (CO2) and nitrogen (N2), by decomposing dead or decaying plant and
animal matter e.g. leaf litter. Terrestrial plants produce c.120 billion tons of organic carbon each
year (Beer et al., 2010) and only a small fraction of this is removed from the ecosystem by
herbivores (Battle et al., 2000). However, recent studies have shown that decomposition in
aquatic ecosystems contribute significantly to global carbon studies (Tranvik et al. 2009; Battin et
al. 2008, 2009).
Almost all of the research on decomposition of detritus has been conducted in aquatic ecosystems
with running water (Wallace, J. B. & Webster, J. R., 1996). Different types of aquatic system may
hold many physical similarities, but they differ significantly in salinity. Freshwater systems have a
salinity of <0.5‰ (parts per thousand), whilst marine environment have a salinity of >30-50‰.
Brackish waters span the salinity between these two. The nature of water in such systems means
that it is constantly flowing, carrying with it a ready supply of organic matter that enables the
continuity of decomposition by the detritivores. All three environments share the medium of
water, and are therefore ideal for a cross comparison of how temperature alters decomposition
rates. This would mean that salinity was the only other compounding factor (Lecerf et al. 2007).
Metabolic rate is a function of both temperature and body mass (Pang, X. et al, 2011). It can be
determined that at higher temperatures organisms will have increased metabolic requirements in
order to sustain their metabolism. And so, in order to maintain the escalation in energy
requirement that occurs at higher temperatures, organisms such as detritivores will show an
increase in the decomposition rate of organic matter (Aerts, R., 2006). Examples of such
detritivores are the Gammarus amphipod species. One species - Gammarus pulex - is found in
freshwater across Europe and Asia, and can usually be discovered under stones, in the mud, or
near detritus. Another type - Gammarus zaddachi - lives in marine or brackish waters. Both
species are largely scavengers that feed on dead plant matter or other decaying organic material.
A meta-analysis of 3 studies suggests that there is a general trend towards increasing
decomposition rate with increasing temperature across different aquatic ecosystems: marine,
freshwater and brackish (e.g. Boyero, L et al., 2011; Pederson, M. O. et al., 2011; Moore, T. L. et
al., 1999; see Table 1). However, the meta-analysis also showed that whilst there have been
numerous studies beyond those reviewed; none studied the concurrent in a systematic fashion.
The present study aimed to compare decomposition rates under two experimental factors:
ecosystem type and temperature. Three levels of each factor were used, in order to determine
whether the effects of warming on decomposition rates were consistent across different
ecosystems, as they appear to be from the available literature. Specifically, it was hypothesised
that: (1) there will be an increase in the decomposition rate of organic matter at higher
temperatures; and that (2) this will be consistent in freshwater, brackish and marine systems.
Table 1
Material and Methods
The hypotheses were tested in a laboratory in order to tightly control the two experimental
factors: ecosystem type and temperature. Carrying out the experiment in a laboratory meant that
all of the ecosystem types could be controlled in one location, while the level of warming of could
also be precisely controlled. Each factor was observed at three levels. The three ecosystems were
freshwater, brackish water, and marine water. These were determined by their salinities freshwater (0‰); brackish water (15‰); marine water (34‰). Each ecosystem type was
incubated at 3 temperatures (5°C, 10°C and 15°C) in temperature control rooms. The rooms were
illuminated for 12 hours, and were dark for the other 12 hours. There were 10 replicates of each
treatment (Figure 1).
2.00 (± 0.02)g of dried alder leaves were precisely weighed using an electric balance scale and
then placed in each tank. 1 litre of the correct salinity of water was measured using a graduated
cylinder and added to each tank. This was 0‰ (freshwater), 15‰ (brackish water) or 34‰
(marine water), depending on the ecosystem type allocated to the group. Three members of the
appropriate Gammarus spp. were then added to each tank. Again, the correct species was
dependent on the ecosystem type assigned (Gammarus pulex for freshwater, and Gammarus
zaddachi for marine and brackish water). The leaves fully immersed in the water to maximise the
possibility for decomposition. The tanks were then incubated in the correct temperature-control
room for 7 weeks. The tanks were aerated with air from an air pump. They were kept in blocks of
ecosystem type to avoid the accidental mixing of water from spatter caused by the air pumps
(Figure 2).
At the end of the incubation
period the water was drained
from the tanks through a sieve
(250 µm mesh size), carefully
retaining the remaining leaf
litter. Forceps were used to
place every piece of remaining leaf litter into envelopes with the same labelling as the plastic
aquaria (ie temperature, ecosystem type, initial weight of leaf litter). Any living Gamarus were
removed, counted and noted on the envelope, and placed back into plastic aquaria containing the
correct water-type. The dead Gammarus were removed before the litter was left to dry out
completely in an 80° oven for 5 days (120 hours). It was re-weighed on an electric scale, and the
weight for all the replicates in all of the ecosystems was recorded. These values were used to
calculate the decomposition rate. The equation to calculate this was: initial mass of leaves – final
mass of leaves/49(days). The results from this calculation were recorded as mg/day-1.
Results
The results of an analysis of variance show that temperature has a significant effect on
decomposition rate (F2,78 = 55.14; p<0.001). Ecosystem type also has a significant effect on
decomposition rates (F 2,78 = 44.53; p<0.001). The effect of temperature on decomposition was
slightly more significant than that of ecosystem type. However, ecosystem type does not
significantly alter the effect of temperature on decomposition rate (F4,78 = 0.41; p=0.800).
Homogeneity of data is desired. The results
showed that there was homogeneity in the data
(Figure 3). The data was also normally distributed
(Figure 4). There is no general interaction between
ecosystem type, temperature, and decomposition
rate (Figure 5).
A Tukey’s test was used to look at the differences
in significance between the difference levels of
each of the variables (Figure 6). At 5°C: A
significant difference was seen between the means
of the marine ecosystems and freshwater
ecosystems, and marine ecosystems and brackish
ecosystems, but not between freshwater and
brackish ecosystems. At 10°C: Again, a significant
difference was seen between marine and
freshwater ecosystems, and marine and brackish
ecosystems, but not between freshwater and
brackish ecosystems. At 15°C: No significant
difference was seen between freshwater and
brackish ecosystems, or brackish and marine
ecosystems. However, a significant difference was
seen between the means of marine and
freshwater ecosystems.
Discussion
The results of the study show that there is a significant effect of temperature on decomposition
rate, with this effect being found to be consistent across all three of the examined ecosystem
types. Although slight differences in significance of response can be seen between the different
ecosystems, these differences are not substantial enough to determine an influential interaction
between ecosystem type and the effect temperature change has on it. This allows for the
conclusion that there was a general lack of association between the two variables - ecosystem
type and temperature, and the responding function – the rate of decomposition. Nevertheless, it is
still possible to infer from the results that decomposition rate increases with increasing
temperature, therefore confirming the two hypotheses of the study. It may be possible to
extrapolate the effects of increasing temperature into different ecosystems, or with different
organisms. The study only used one type of organism (Gammarus spp.) and was limited to only
three ecosystem types, all of which were aquatic. To be able to generalise these findings to other
systems would further better the ability of models to predict the role of climate change (in
particular warming) in altering different processes within an ecosystem (Gunawardhana, L. N. &
Kazama, S., 2011).
The idea that an increase in temperature - in particular one triggered by climate change - will
have a corresponding result on various ecosystem functions has been modelled often (Kardol, P.
et al, 2010; Pedersen, M. O. et al, 2011; Beveridge et al, 2010). The results of the current
experiment, in which a positive relationship between increasing temperature and decomposition
rate can be seen, are mirrored in the results of several studies. One such example by Ferreira &
Chauvet (2011) approaches the idea of temperature working in synergy with dissolved nutrient
concentrations in order to effect litter decomposition in woodland streams. They found that all
biological variables – including decomposition rate – were stimulated when both of these factors
increased. The results of the study are supported by those of the current experiment, and also
suggest that other characteristics of an ecosystem play a significant role in the ecosystem’s
response to climate change.
Upon observing the tanks after their 48-day period in the temperature controlled rooms, it was
noted that there was a poor survival rate in the amphipods. This is problematic as it is unknown
how far into the experiment they started to die. In many cases the bodies of the amphipods
weren’t found, suggesting that decomposition occurred in any case, without the presence of
Gammarus spp in the ecosystem. This unconsidered decomposition is likely due to the effects of
microbial detritivores undoubtably residing in the tanks, which most likely entered the ecosystems
via the water medium, or attached to the leaves used. Gessner et al (1999) suggested that the
view of litter decomposition in freshwaters needed to be improved. They proposed that the
processes could be broken down into three distinct stages. The current study addresses only one
of these stages; the final stage of decomposition facilitated by detritivores invertebrates such as
Gammarus, and ignores the second stage of litter breakdown during which microbial decomposers
enhance the palatability of the little for the invertebrate organisms. The existence of such
organisms implies that the study actually measured the effects of both macro- and microdecomposition.
In many ways this study is extremely limited. One such limiting factor that is rectifiable, is tied to
the fact that the amount of decomposition measured could well have been due to the additional
effects of the amphipods and the microbial decomposers. A way to remedy this for the future
would be to create a control ecosystem in which only the microbial decomposers exist, and
observe the average decomposition that occurs over the same amount of time. The rate of
decomposition calculated in this control could then be subtracted from the rates seen in the tanks
containing both Gammarus and microbes (i.e. the ecosystems used in the current study). The
resultant data would then allow the relative contributions of both the Gammarus and the microbial
decomposers to be calculated and separated out.
Although the study is highly replicable (as seen in the data) because it was conducted in a
laboratory, the mesocosms used were poor imitations of their real-life counterparts meaning the
study lacked realism. It therefore makes it difficult for the results to be used as a model that can
be applied to naturally-occurring ecosystems. In order to create such a model, several factors
must be observed in the natural ecosystems over a long period of time, including the effects of
climate change on biological characteristics of aquatic ecosystems such as lakes. These
observations should be made over a wide range of ecosystem types - much like the ones observed
in the current laboratory study - and the heterogeneity of the responses should be surveyed. This
will enable long-term modelling of the effects of warming on many ecosystem services as well as
the effects on decomposition rates.
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