Vertical foodweb structure of freshwater zooplankton assemblages estimated by stable nitrogen isotopes

Abstract

Although theoretical foodweb models predict the presence of only three to four trophic categories, estimation of "potential" vertical foodweb structure from species lists and inferred feeding interactions suggest that as many as 7 trophic categories can occur in the pelagic foodwebs of North American glaciated lakes. A compilation of data on the nitrogen isotopic composition of zooplankton from 46 Canadian Shield lakes suggested the average existence of one "realized" trophic category in addition to that of filterfeeding, herbivorous cladocerans. When phytoplankton, planktivorous invertebrates, and plantivorous and piscivorous fish are included, the vertical foodweb structure in the pelagic zones of these lakes are greater than those hypothesized from some theoretical models.

Productivity and heterotrophy influences on zooplankton delta13C in northern temperate lakes

“…In all cases, these size fractions represent macrozooplankton (del Giorgio & Gasol 1995). France et al (1996) present stable nitrogen isotope data for these same zooplankton samples.…”

“…Therefore, as occurs in the understory layer of many forests (Schelser & Jayasekera 1985, Medina et al 1986, Da Silveira et al 1989, France 1995d, photoassimilation of this recycled CO2 will produce lower 613C values of autotrophs (Rau 1978, McCabe 1985, Hollander & McKenzie 1991, del Giorgio & France 1996.…”

“…Planktonic heterotrophs are capable of utilizing some of this carbon (Jones 1992) which maintains the relatively constant (-29 to -26%0) 6' " signature of temperate terrestrial vegetation. The proportion of allochthonous detrital organic matter in the total carbon pool tends to decline with lake productivity (Jones 1992, del Giorgio & Gasol 1995, del Giorgio & France 1996, so that the relative importance of this allochthonous pool to the C isotopic values of the plankton community should also decline with lake productivity. Thus, there are 2 fundamental but contrasting processes which might shape the F13C values of freshwater plankton: (1) increased utilization of respiratory CO2 in oligotrophic lakes, leading to 13C-depleted phytoplankton; and (2) increased utilization of 13C-enriched allochthonous organic matter which should lead to I3c-enriched planktonic heterotrophs.…”

“…First, the 6I3C value of phytoplankton can seldom be reliably measured because of dilution with the large detrital component present in many lakes (Hamilton & Lewis 1992, del Giorgio & France 1996. Second, phytoplankton are not the sole food source to zooplankton in lakes, and planktonic heterotrophs together with detritus can often form a variable and yet unquantified fraction of zooplankton diet (Jones 1992).…”

Leaves as “crackers”, biofilm as “peanut butter”: Exploratory use of stable isotopes as evidence for microbial pathways in detrital food webs

“…1). This potential scenario is analogous to that which occurs within the pelagic food web wherein herbivorous zooplankton display σ 15 N values of about +6‰ (France 1994), three units above those of their phytoplankton food source (France et al 1996). In contrast, terrestrial (non-coastal, non-xeric, nonagricultural) plants range in σ 15 N from -7 to +9‰ with a mean of 0‰ (France 1995b).…”