Poor Having that in mind, undergoing processes within

Poor water source sink relationship within chalk streams, implications for ecosystem structure
and function.
Introduction
The ecosystem consists of living organisms (biotic compound), their habitat (abiotic compound), and
exchanges, transformations of matter and energy between them. The combination of biotic and abiotic aspects
of an ecosystem produces the structure of the ecosystem, where the energy and nutrient flow is a function of
the ecosystem. Ecosystem structure is the main aspect in order to maintain healthy ecosystem function and
providing good services for all living organisms.
Rivers make up only 0.01% of all Earth’s water budget. Despite this, the importance of lotic ecosystem on a
global scheme is highly valuable. Because of hydrological connectivity, rivers and streams are fundamental
for global water cycle; flowing water provide pathways for biogeochemical exchange within aquatic and
terrestrials ecosystems, unifies water bodies and oceans. Due to its dynamics, a lotic ecosystem is sculpting
surface, creating habitats and, being strongest erosion agent, provides geochemical materials to all systems.
Having that in mind, undergoing processes within fluvial ecosystems has a major impact not only on the scale
of particular river system but also has an overall, global importance. A lotic system has to benefits for
humans: direct exploitation of rivers (food, fresh water, etc.) and indirect (climat change, temperature control,
etc.), by providing nutrients and organic matter to terrestrial systems, in order to maintain healthy
biodiversity which sustains a healthy environment for humanity (Cardinale et al., 2012).
Like within all ecosystems, the main function of rivers and streams are energy transportation and nutrient
turnover. Although, due to hydrological connectivity the processes within a fluvial structure, river
functioning is tightly related with a surrounding terrestrial structure. Autochthonous, allochthonous subsidies,
processing and retaining organic matter; all combined, creates a complex, unified functioning system, where
the flowing water is highly important.
The differences in physical characteristics, such as river flow and depth are supporting heterogeneous
environment within an ecosystem. Flow is one of the factors to determine primary features of the river bed
(sediment granulometry, deposition of sediment and organic matter, oxygen concentration, etc.), which
underpin primary conditions for fauna and flora communities. Combination of abiotic and biotic structure
within the habitat, maintain ecosystem functioning in a different way; production and respiration, nutrient and
energy flow, all this function are dependent on the structural aspects of the ecosystem.
The river Cray was surveyed as a model system in order to understand, how differences of underlying
ecosystem structure supports differences in the functioning of an ecosystem.
The heterogeneity within the river Cray is visually noticeable; three different patch types (microhabitats)
were surveyed: submerged- mostly covered by macrophytes, area with relatively slower flow, main channel –
area of clean, open running water, mostly with an absence of macrophytes and higher flow over large caliber
substrate, marginal strip – areas closest to the bank of the river, depositional areas, with an increasing
concentration of sediment deposition, slowest water flow. The aim of the research is to see if the variations
within ecosystem structure can lead to differences in function in order to provide ecosystem services. We
focused on measuring nutrients NO2

, NO3

, NH4
+
, and PO4
3-
, in order to detect how a difference in
microhabitat structure is supporting the provision of key nutrients, which maintain healthy ecosystem
functioning to provide ecosystem services.
Methods
Study site
The river Cray (latitude 51.424294; longitude 0.12654262) was chosen as a model system for this study. The
river rises at natural ponds in Orpington are in Bromley, southern England. The upstream catchment of the
river is very highly urban, here the river is modified and flow within artificial channels and culvert sections.
Middle and downstream catchment are in a more natural territory, here the river flows through semi-natural
parkland in Footscray Meadows and Hall Place (Environmental Agency, 2017).