Consequences of detritus type in an aquatic microsystem: effects on water quality, micro-organisms and performance of the dominant consumer

Abstract

1. Variation in detritus quality and quantity can have significant effects on aquatic invertebrate food webs. Allochthonous inputs of detritus are the principal energy source for organisms in aquatic tree hole microsystems. We compared the effects of two major detritus types found in tree holes, senescent leaves (Sugar Maple and White Oak) and invertebrate carcasses (dead adult fruit flies and crickets), on several water quality characteristics of laboratory microcosms as well as on mass, survival and population performance of the dominant tree hole consumer, Ochlerotatus triseriatus (Diptera: Culicidae). To date, no study has documented the effects of animal detritus in tree hole microsystems or on resident consumers.2. Aquatic environments receiving invertebrate carcasses had significantly greater total nitrogen, total reactive phosphorus and higher pH, than leaf-based environments. Decay rate of invertebrate carcasses was greater compared to leaf material. Consumption of O(2) by micro-organisms increased with increasing detritus amounts, but we detected no difference between detritus types.3. Ochlerotatus triseriatus larvae grew faster in animal-based treatments, and mean mass of larvae was significantly greater when more animal detritus was used. The effect of animal-based treatments on larvae translated into higher performance for adults, which were three times heavier than counterparts from plant-based containers. Survivorship and estimated population growth rates were significantly greater for O. triseriatus reared on animal-based versus plant-based detritus.4. We hypothesise two mechanisms for the pronounced effect of invertebrate carcasses on mosquito performance relative to that associated with leaf detritus: (i) invertebrate carcasses decompose more quickly and release nutrients more effectively into the aquatic environment; or (ii) O. triseriatus larvae may directly ingest nutrient-rich components of invertebrate carcasses. Because even relatively small animal detritus additions can have strong effects on O. triseriatus populations, studies need to be conducted to explore the overall role of animal detritus in tree holes in nature.

Fig. 1

Mean (±1 SE) of the significant treatment (detritus type × amount (g)) by day effect on water chemistry measurements from laboratory microcosms. (a) pH, (b) conductivity (μMHOS cm⁻¹), (c) total nitrogen (mg L⁻¹) and (d) tannin (mg L⁻¹). Day corresponds to the day of measurement.

Fig. 2

Mean (±1 SE) total phosphorus (mg L⁻¹) values among different detritus type by amount (g) combinations. Different letters indicate significant differences among means.

Fig. 3

Per cent decay (mean ± 1 SE) of initial detritus amounts (g). Significant treatment (upper figure) and time (lower figure) effects are illustrated. Means in the upper figure are calculated across all sample days, whereas means in the lower figure are calculated across all detritus types by amounts within each sample day. Day corresponds to the day of measurement. Different letters indicate significant differences among means.

Fig. 4

Bivariate plot of the significant effect of detritus type and amount (g) on mean larval instar (±1 SE) and mean larval mass (±1 SE) on day 12 of experiment 2. Detritus types are: C, cricket; D, Drosophila; M, Maple; O, Oak.

Fig. 5

Performance of adult Ochlerotatus triseriatus for experiment 3. (a) Mean proportion survival, (b) mean adult female mass (mg) and (c) mean estimated rate of increase (λ′). All means are ±1 SE. The dotted line at λ′ = 1 corresponds to zero net growth of the experimental populations. Only one female was produced from 0.05 g Oak containers and thus a value for λ′ could not be generated.