Surfaces light zone which in turn influences the

Surfaces of buried substances
within rivers and streams are often covered by colonies of micro-organisms
(which may be any of fungi, bacteria, algae) as well as other unicellular
organisms in cluster of polysaccharide (Wetzel, 1983). They are collectively
reffered to as ‘biofilms’ and are rapidly influenced by slight changes in the
conditions within their immediate environment such as fluctuations in amount of
water present in the river or stream (Steinman & McIntire,  1990).Hence, very short life span. In the
formation of these micro-organism assemblages, process such as adhesion, accumulation and detachment of cell of microbes
all occur one after another (Jenkinson & Lappin-Scott, 2001).

Biofilms have been studied to play
vital roles in the biogeochemical cycles and metabolism of streams and rivers (Romani, et al., 2013).Those found in rivers as
well as streams are subjected to quite a number of environmental factors which
vary from nutrient availability to flow and chemistry of water, light regime
and the availability of dissolved organic matter (DOM) (Roberts, Sabater, & Beardall, 2004). It is these factors that
result in the structure and function of biofilm been dependent upon light,
temperature and presence of nutrients among others which ultimately influences
biofilm thickness, its function and what it is made up of, as well as the
relative abundance of autotrophs/heterotrophs (D?az
Villanueva, Font, Schwartz, & Romani, 2011). Photosynthetic
organisms such as diatoms and green algae are seen to dominate the fold of
biofilms in the presence of light (Peterson, 1996), whereas without light or in
very low light conditions, bacteria dominate biofilms (Blenkinsopp & Lock, 1994).
Lock, Wallace, Costerton, Ventullo, &
Charlton, (1984) explained that the level at which 1/100 light
penetration determines the depth of the light zone which in turn influences the
change of dominance of biofilm by autotrophs to heterotrophs.  Romani, et al.,
(2004) explained that at the level of river ecosystem, the effect of
biofilm on DOC is mainly controlled by quality and quantity of organic carbon
as well as biofilm surface. In the same vein, biofilms in some rivers (Mediterranean)
are greatly affected by both seasonality and fluctuations. As a result, floods
and droughts tend to affect the available water in these rivers, the nutrients
that aid growth of microbes as well as the DOM (in terms of quality and
quantity) (Romani, et al., 2013).

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