Nitrogen effects on the pelagic food web are modified by dissolved organic carbon

Abstract

Global environmental change has altered the nitrogen (N) cycle and enhanced terrestrial dissolved organic carbon (DOC) loadings to northern boreal lakes. However, it is still unclear how enhanced N availability affects pelagic food web efficiency (FWE) and crustacean zooplankton growth in N limited boreal lakes. Here, we performed in situ mesocosm experiments in six unproductive boreal Swedish lakes, paired across a DOC gradient, with one lake in each pair fertilized with N (2011: reference year; 2012, 2013: impact years). We assessed how zooplankton growth and FWE were affected by changes in pelagic energy mobilization (PEM), food chain length (phytoplankton versus bacterial production based food chain, i.e. PP:BP), and food quality (seston stoichiometry) in response to N fertilization. Although PP, PEM and PP:BP increased in low and medium DOC lakes after N fertilization, consumer growth and FWE were reduced, especially at low DOC-potentially due to reduced phytoplankton food quality [increased C: phosphorus (P); N:P]. At high DOC, N fertilization caused modest increases in PP and PEM, with marginal changes in PP:BP and phytoplankton food quality, which, combined, led to a slight increase in zooplankton growth and FWE. Consequently, at low DOC (<12 mg L^-1), increased N availability lowers FWE due to mismatches in food quality demand and supply, whereas at high DOC this mismatch does not occur, and zooplankton production and FWE may increase. We conclude that the lake DOC level is critical for predicting the effects of enhanced inorganic N availability on pelagic productivity in boreal lakes.

Keywords: Boreal lakes; Global change; Nitrogen availability; Trophic transfer efficiency; Zooplankton.

Fig. 1

Conceptual model illustrating the predicted response of the boreal pelagic food chain to increased dissolved inorganic nitrogen (DIN) availability in lakes with (a) low dissolved organic carbon (DOC) and (b) high DOC. Thickness of arrows represents pathway strength. Other abbreviations are total pelagic energy mobilization (PEM), total zooplankton production (TZP), dissolved inorganic carbon (DIC), and phosphorus (P)

Fig. 2

Volumetric estimates (± standard errors) of a primary production (PP), b total bacterial production (BP), c total pelagic energy mobilization (PEM = PP + BP), and d food chain length shown as PP:BP during the mesocosm experiment (July–August, n = 3) in the control lakes (NoN) and N lakes (+N) before (‘Before’, 2011), and after N fertilization (‘After’, 2013) across the DOC gradient (white low DOC, gray medium DOC, black high DOC). Fertilization effects are presented as ‘Δ After’, illustrating the difference between respective values before and after N fertilization

Fig. 3

Seston stoichiometry (± standard errors) of a N:P, and b C:P molar ratios during the mesocosm experiment (July–August, n = 3) in control lakes (NoN) and N lakes (+N) before (‘Before’, 2011), and after N fertilization (‘After’, 2013) across the DOC gradient (white low DOC, gray medium DOC, black high DOC). Fertilization effects are presented as ‘Δ After’, illustrating the difference between respective values before and after N fertilization

Fig. 4

Changes in a seston carbon (µg L⁻¹), b seston N:P and c C:P ratios during the whole growing season in response to N fertilization in lakes across the DOC gradient (black fertilized lakes, n = 6, i.e. N lakes in 2012, 2013; white non-manipulated lakes, n = 12, i.e. control lakes in all years, N lakes in 2011) with regression lines (thick line fertilized lakes, thin line non-manipulated lakes) and 95% confidence intervals (dashed)

Fig. 5

a Community composition of zooplankton, and b total zooplankton biomass (± standard errors) during the mesocosm experiment (July–August, n = 3) in control lakes (NoN), N lakes (+N) before (‘Before’, 2011), and after N fertilization (‘After’, 2013) across the DOC gradient. Fertilization effects are presented as ‘Δ After’, illustrating the difference between respective values before and after N fertilization

Fig. 6

Volumetric estimates (± standard errors) of a total zooplankton production (TZP), growth rates of b Calanoida, c Cyclopoida, d Bosmina, and e Ceriodaphnia, and f food web efficiency (FWE) during the mesocosm experiment (July–August, n = 2) in control lakes (NoN) and N lakes (+N) before (‘Before’, 2011), and after N fertilization (‘After’, 2013) across the DOC gradient (white low DOC, gray medium DOC, black high DOC). Fertilization effects are presented as ‘Δ After’, illustrating the difference between respective values before and after N fertilization

Fig. 7

Linear regression between total zooplankton production (Tot. Zoopl. Production) and a seston N:P and b C:P, and between food web efficiency (FWE) and c seston N:P and d C:P during the time frame of the mesocosm experiment (July–August, n = 3) including 95% confidence intervals (dashed). Note log scale