Gutter Subsidy

Definition: The added organic loads to stream ecosystems originating from the “rain” of organic matter in urbanized upland landscapes, transported via engineered drainage networks.  

Explanation: These additional, stormwater mediated loads to streams from upland leaves and other organic matter carried by streets and gutters dwarf natural sources (i.e. riparian areas around streams) and represent a significant “gutter subsidy” to stream food webs (see “upland riparian zone”.)  Conversely these exported upland loads also represent subtractions from the natural, normally tight, cycling of organic matter and nutrients between leaf litter, soils and trees and other vegetation.  It is also likely that a significant part of these organic loads originate from animal waste and other trash from streets and alleys.

Example: Tree leaves and other organic matter (particulate and dissolved leachate) fall into, or are blown into drainage swales or gutters during the growing season (greenfall) or in the autumn pulse of abscised leaves; stormwater runoff then carries them into dense networks of pipes which flow into streams.

Figure 1: Organic matter processing in roads; Leaves (CPOM), fragments (FPOM) and leachate in the Dead Run and Baisman Run catchments.  Upper left: Wetted, macerated leaves on road crossing Baisman Run; Lower left: Leaves in Dead Run gutter which have leached their organic matter into recent runoff gutter puddles; Upper right: Leaves blown or washed into a gutter and storm drain inlet leading into Dead Run.

 

Why Important: In urban areas leaf litter represents an important “gutter subsidy” to stream food webs and also provides habitat for aquatic biota.  It likely affects other pollutants (e.g., binding metals, disinfection by-products) and so is a key component of the urban aquatic ecosystem. 

Contributed by BES Co-PI Kenneth T. Belt