Environmental Change in Marine Systems

In the first part of the introduction to this module, we examined key features of the marine environment. In this second, part, we look at differences between marine and terrestrial systems. This includes looking at aquatic freshwater systems, as these have similarities, and differences, to both marine and terrestrial systems.

2. Difference between marine and terrestrial/aquatic systems

Some differences between marine and terrestrial systems are due to differences between basic properties of air and water.

Water has a high specific heat: as a consequence, changes in temperature are damped – occur more slowly and may have a smaller magnitude – in aquatic environments than in terrestrial environments (although this does depend upon the size of the water body).

Light penetrates much further through air than through water: as a consequence, photosynthesis, and so plant growth, is limited in deep water (see: the aphotic zone). In the water is particularly turbid, plants may only be able to grow in the top few metres. The limited light penetration also means that there is limited heating of deeper waters (unless there is submarine vulcanism; and see: hydrothermal vents).

One of the readily apparent differences between marine and terrestrial organisms is in the taxa present: for instance, many animal phyla are found only in marine environments.

Grassle et al. (1990) stated that: "If we are to be able to predict the future consequences of environmental change – e.g., global wanning in particular climatic zones – it is essential that comparative studies be undertaken so that regional differences may be properly documented, and so that the natural and/or anthropogenic mechanisms controlling them are understood" (italics added). Among the differences they were referring to were differences between marine and terrestrial systems. Read the article, "Marine biodiversity and ecosystem function", up to, but not including, the section on "Programme development".

It is important, however, to not overstate the differences. Remember that in Module 1 (Physical Environment), it is noted that the processes of convection and conduction occur in both aerial and aquatic environments.

Dawson and Hamner (2008) state: "Nonetheless, it has been argued repeatedly that the geography of evolution differs fundamentally between marine and terrestrial taxa." They nonetheless argue: "The fluid mechanics of marine and terrestrial systems are surprisingly similar at many spatial and temporal scales." Read their article, "A biophysical perspective on dispersal and the geography of evolution in marine and terrestrial systems", up to, but not including, section 3.3.

Dawson and Hamner (2008) give some of the differences between the environments, as they relate to air and water:

"For example, surface seawater is approximately 96.5% water and 0.5% oxygen; in contrast, air is utmost 1.5% water (1 atm, 20°C, 100% specific humidity) and approximately 20% oxygen. The specific heat capacity of seawater (approx. 3850 J kg−1 K) is approximately four times that of air (approx. 1030 J kg−1 K). Sound travels faster (approx. 4.39 times, at 20°C), electrical resistivity is greater (approx. 1016 times) and oxygen replenished slower (approx. 10−4 times) in seawater than in air. Two of the most apparent and commonly noted differences are that seawater is more dense and viscous (at 20°C, 34.84‰, 1 atm: density ρ =1024 kg m−3 and dynamic viscosity μ=1.072×10−3 kg m−1 s−1) than air (at 20°C, 1 atm: ρ=1.205 kg m−3 and μ=18.08×10−6 kg m−1 s−1; Vogel 1981)."

These differences mean that organisms may experience different kinds of stresses in marine environments, compared to terrestrial environments. For instance, the slower rate at which oxygen is replenished means that animals in marine environments may sometimes be stressed by low levels of available oxygen, something which tends to be unusual in terrestrial environments. The greater density of the water provides support for organisms, so structural supports (e.g. skeletons) may be reduced, but flowing water may subject them to greater physical stresses.

Terrestrial organisms have evolved mechanisms to conserve water, which is often scarce, but this is not a problem which marine organisms face, at least directly. Indirectly, marine organisms may experience osmotic stress as their membranes (e.g. skin, gill surface) are often permeable to water. If the concentration of ions in the organism's tissues is lower than that of surrounding seawater, then water will tend to flow across the semipermeable membrane (e.g. gill surface) into the surrounding environment. This situation is described as hypertonic: a greater concentration of ions in the environment, compared to within the organism. Many marine organisms, however, are isotonic with their environment.

Cole (1940) reported on an early study of the relationship between the composition of seawater and the body fluids of marine animals. His results indicated that the body fluids of several of the groups of animals examined were isotonic with seawater but others were not. You can read the article online: "The composition of fluids and sera of some marine animals and of the sea water in which they live". The critical results are in Table 1 (I). If the value in the –∆ column for the animal is similar to the value for seawater, then the animal's body fluids are approximately isotonic to seawater. Results for particular ions are in Table 2 (II): values close to 1.0 indicate that the concentration of that particular ion is similar in seawater and in the animal's fluids.

Many mangroves are exposed to potential desiccation stress because the soil in which they grow may have high levels of salt. Organisms which experience osmotic stress must expend energy to maintain their internal osmotic balance within tolerable limits.

Intertidal environments

Intertidal environments are unique as the organisms inhabiting them experience alternating periods of submersion and exposure. As a consequence, these organisms must cope with the stresses of both aerial and aquatic environments. We examine this in more detail in the next topic in this module (Stress).

Read some introductory information about intertidal environments (also referred to as littoral) here: http://en.wikipedia.org/wiki/Intertidal_zone.
Note: Module 1 (Physical Environment) introduced the ideas of the microclimate and the nanoclimate. These are, of course, also relevant to marine environments and, in particular, to intertidal environments. In intertidal environments, there may be dramatic changes in the microclimate and nanoclimate in a particular location – for instance, the microclimate in the middle of the shore and the nanoclimate under a boulder in the middle of the shore – between when the tide is in, and when it is out.

Investigating intertidal environments and habitats requires some understanding of the tides.

Read about the characteristics of tides and how they generated: http://en.wikipedia.org/wiki/Tide (read the first couple of sections).
Note: Tidal predictions for different locations around Australia are available on-line: here for the Northern Territory. (You can even download a Firefox extension which calculates the tide for your current location and shows it in the status bar. Note that the value reported is approximated and should not be relied upon for critical applications.)

Aquatic freshwater systems

Freshwater environments differ from marine environments in several ways. First, obviously, the water has a low concentration of ions (some aquatic environments are hypersaline): as a consequence, organisms inhabit a hypotonic environment and those with semipermeable membranes usually require special mechanisms to maintain the correct osmotic balance in their tissues and body fluids. Second, these smaller bodies of water may exhibit greater, and more rapid, changes in environmental conditions – such as temperature and oxygen levels – than the oceans.

Activity 8

Complete the activity to test your understanding of these issues.

References

The key sources for this first section are listed below.

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