Landscapes Forest and Global Change - ESA - Escola Superior ...

P.C. Goebel et al. 2010. How important are riparian forests to aquatic foodwebs in agricultural watersheds
130
webs, which in turn, provide the assimilative mechanisms for nutrients including nitrogen and
phosphorus (Vannote et al. 1980; Minshall et al. 1985). In most cases, the processing of
nitrogen, phosphorus, and organic matter (carbon) is strongly influenced by channel
morphology (e.g., sinuosity and connection to an active floodplain), habitat characteristics (e.g.,
large wood that traps leaves and slows downstream movement) and diversity and structure of
the aquatic community. A diversity of organisms in the aquatic food web sequesters nutrients
and slows their downstream transport. In contrast, in managed and degraded headwaters, large
proportions of inorganic nutrients enhance the growth of algae, which lowers levels of dissolved
oxygen, consequently limiting the survival of more desirable invertebrate and vertebrate
species. The degree to which this assimilative capacity remains intact is particularly critical in
predominantly agricultural watersheds where riparian forests have been removed and nutrient
loadings to streams tend to be high. In the absence of organic carbon inputs, as is encountered
when riparian vegetation is removed, the capacity of streams to process and retain nutrients can
be lowered significantly (Malanson 1993). In these systems, it remains unclear whether instream
production is an alternative energy source to woody riparian vegetation that structures
aquatic food webs.
In this paper, we examine the relationships between basal resources and consumers in small and
often isolated riparian forest fragments within a larger agricultural landscape. To accomplish
this, we utilize stable isotope analysis to quantify the use of terrestrial and aquatic sources of
organic matter by consumers and explore how the presence of riparian forest fragments can
affect the source of energy and its availability to aquatic consumers in headwater streams. Use
of carbon (δ 13 C) and nitrogen (δ 15 N) stable isotopes is a powerful tool to evaluate trophic
relationships in aquatic food webs (Peterson and Fry 1987). Because the δ 13 C composition of
animals correspond closely to those of their food sources, it is possible to evaluate the
contribution of various sources of carbon to the energy flux of aquatic systems. Such connection
between food sources and the consumers can only be done when sources have distinctive δ 13 C
ratios (Hamilton et al. 1992). It has also been shown that in aquatic systems, allochthonous and
autochthonous sources of carbon tend to generally have different δ 13 C signatures, providing a
tool to evaluate their incorporation into the aquatic food web. δ 15 N isotope ratios, in contrast,
become more enriched at successive trophic positions.
2. Methodology
2.1 Study area, site selection, and field sampling
We conducted our research in the Sugar Creek watershed, located in northeastern Ohio, USA.
The Sugar Creek watershed covers 922 km 2 and is dominated by different types of agriculture,
including conventional row-crop production and traditional Amish farming practices (Stinner et
al. 1989). The landscape is best described as fragmented, with small isolated riparian forests
and woodlots typically comprising less than 10% of the total watershed area. Within this
watershed, we selected nine stream reaches dominated by small forested riparian areas and ten
stream reaches dominated by non-forested riparian areas.
At each site, we collected three categories of organic materials: (1) autotrophs from the channel
(i.e., macroalgae, macrophyte, and epilithon) and the riparian area (i.e., herbaceous vegetation
and leaves from trees), (2) detrital material (i.e., coarse particulate organic matter or CPOM,
fine particulate organic matter or FPOM, and dissolved organic matter or DOC), and (3)
consumers (both macroinvertebrates and fishes). Riparian trees were only sampled at forested
sites as they were mostly absent from non-forested sites. All samples were collected in the
spring of 2008 once from each location.
Forest Landscapes and Global Change -New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.