Terrestrial subsidies to lake food webs: an experimental approach

Abstract

Cross-ecosystem movements of material and energy are ubiquitous. Aquatic ecosystems typically receive material that also includes organic matter from the surrounding catchment. Terrestrial-derived (allochthonous) organic matter can enter aquatic ecosystems in dissolved or particulate form. Several studies have highlighted the importance of dissolved organic carbon to aquatic consumers, but less is known about allochthonous particulate organic carbon (POC). Similarly, most studies showing the effects of allochthonous organic carbon (OC) on aquatic consumers have investigated pelagic habitats; the effects of allochthonous OC on benthic communities are less well studied. Allochthonous inputs might further decrease primary production through light reduction, thereby potentially affecting autotrophic resource availability to consumers. Here, an enclosure experiment was carried out to test the importance of POC input and light availability on the resource use in a benthic food web of a clear-water lake. Corn starch (a C(4) plant) was used as a POC source due to its insoluble nature and its distinct carbon stable isotope value (δ(13)C). The starch carbon was closely dispersed over the bottom of the enclosures to study the fate of a POC source exclusively available to sediment biota. The addition of starch carbon resulted in a clear shift in the isotopic signature of surface-dwelling herbivorous and predatory invertebrates. Although the starch carbon was added solely to the sediment surface, the carbon originating from the starch reached zooplankton. We suggest that allochthonous POC can subsidize benthic food webs directly and can be further transferred to pelagic systems, thereby highlighting the importance of benthic pathways for pelagic habitats.

Importance of terrestrial subsidies for estuarine food webs in contrasting East African catchments

“…Impacts such as deforestation, urbanisation, agriculture and flow regulation can directly affect the energetic connectivity between land and rivers, and these impacts can be transported downstream to the estuaries. Although much research on the importance of terrestrial subsidies has been conducted on temperate lakes (e.g., Bartels et al 2012), rivers (e.g., Kendall et al 2001) and to a lesser extent on estuaries (e.g., Lewis 2002, Sakamaki et al 2010), information on the main sources of energy for tropical estuaries is still lacking. Processes in these areas are likely to be very different to those in temperate systems given the much higher seasonality, productivity and biological diversity of their waters, and the types of human impacts they are subjected.…”

Response to Comment: Terrestrial support of pelagic consumers in unproductive lakes—Uncertainty and potential in assessments using stable isotopes

“…These shortcomings are especially problematic in unproductive lakes where the phytoplankton are dominated by small and mixotrophic species, and where the internal photosynthesis is low compared to the input of terrestrial OM (Algesten et al 2004;Jansson et al 2008). Several alternative analyses and approaches have been tested to overcome these methodological limitations, including compound-specific analyses of phytoplankton biomarkers (Pace et al 2007; Van Den Meersche et al 2009;Berggren et al 2014), manipulation of phytoplankton d 13 C by addition of 13 C-labeled dissolved inorganic carbon (Pace et al 2004;Taipale et al 2008), addition of 13 C-enriched OM (Karlsson et al 2007;Bartels et al 2012), and various mass balance and modeling approaches (Marty and Planas 2008;Mohamed and Taylor 2009;Berggren et al 2010). Still, a generally applicable method is lacking, implying that the problems with assessing d 13 C of phytoplankton is a major limitation in the use of d 13 C for estimating allochthony with the accuracy needed for detailed understanding of food web dynamics.…”

Young, Old, and Weathered Carbon-Part 1: Using Radiocarbon and Stable Isotopes to Identify Carbon Sources in an Alkaline, Humic Lake

“…Debate remains as to processes by which dissolved and particulate organic carbon (DOC and POC, respectively) are utilized by lake heterotrophs in addition to the fraction of each consumed (Bartels et al 2012;de Kluijver et al 2012;Wilkinson et al 2013). The biochemical composition and resulting nutrient value (biochemical quality) of DOC or POC governs selection as a food source, yet stable isotope end members are often insufficiently distinct to distinguish them as carbon sources to aquatic consumers.…”