Hills and Streams: An Ecology of Hong Kong

6. Land and Water

6 Land and Water The stream and its valley Inhabitants of streams and rivers must contend with the unidirectional flow of water from the upper reaches down to the sea. This has the consequence that events upstream affect those downstream, so that running-water habitats do not consist of a series of distinct autonomous sections but rather an interconnected continuum. A second distinctive feature of streams is that they are embedded in terrestrial landscapes and cannot be considered in isolation from their surroundings . Water entering a stream or river has flowed over the land surface or percolated through the soil so that its dissolved and suspended loads reflect, to some degree, the nature of the landscape. The edge to area (or volume) ratio of lotic (running water) habitats is a good indicator of terrestrial influences on the aquatic environment. The combined extent of both banks along a stream course presents a large expanse over which lateral movement of material into a relatively small aquatic habitat can take place. These edge- or bank-effects must be taken into account when conservation and management strategies for streams are formulated. While a sufficiently large woodland patch may be more or less independent of surrounding habitats (except near the margins), the extent of the banks of streams and rivers and the fact that they drain the surrounding land ensures a close link between streams and their valleys. Maintaining the integrity of land-water in79 Hills and Streams: An teractions is therefore a necessary element of stream conservation and management. An additional link between the stream and its valley arises because the water in the channel is not confined within the banks but is continuous with the ground water underlying the surrounding land; this overlap of land and water is most extensive in lowland flood-plain rivers with alluvial valleys. Thus, streams are related to their surroundings in three spatial dimensions: interactions are longitudinal along the stream tributary network to the estuary, vertical with the groundwater, and lateral with the banks, soil water and riparian vegetation. There is an important fourth dimension to consider in understanding stream communities - that of time, with seasonal (e.g., monsoons, floods) and longer-term (e.g., climatic transformation, acidification) changes having short-term behavioural or longer-term evolutionary effects. Despite the four-dimensional nature of stream ecosystems, the unidirectional flow of current is the major factor underlying the distribution and abundance of lotic organisms. The stream can be thought of as a one-way channel along which water, as well as suspended and dissolved materials, both organic and inorganic, travel. The persistence of stream communities depends on the ability of the constituent organisms to collect and retain transported materials which serve as food for animals, as a substratum for microbes, or as nutrients for plant growth. At the risk of oversimplifying, many attributes of the stream community will reflect the balance between the biological potential for collection and retention of organic matter and nutrients, and the downstream export of such material by the current. The influence of physical factors Stream organisms are influenced by a range of interacting abiotic factors, the most important of which are current velocity, flow pattern and discharge, substratum characteristics (including particle size and unevenness), temperature, and dissolved oxygen. The nature of the physical environment in streams reflects the effects of current velocity on the movement of inorganic particles, and the composition of the stream bed in a given section (e.g., the proportions of sand, gravel and boulders) affects the type of organisms which are able to live there. Because the physical characteristics of a stream change along a 80 Land and Water continuous gradient from headwaters to mouth, so too will the stream community alter and thereby exhibit a pattern of longitudinal zonation. In addition, however, there will be variation or 'patchiness' in the physical habitat at any single point along the stream course. For example, where the stream bed is composed mainly of boulders, smaller rocks and gravel will fill up the interstices of the substratum between them. Further complexity is added because sand and fine particles accumulate in isolated pockets sheltered from the current along the stream margins and under, or in the lee of, large rocks and boulders. There is a pattern to some of this substrate variation because physical characteristics change markedly and in a predictable fashion across the stream bed. Current velocities are highest in midstream...