Iron Constraints on Planktonic Primary Production in Oligotrophic Lakes

Abstract

Phototrophic primary production is a fundamental ecosystem process, and it is ultimately constrained by access to limiting nutrients. Whereas most research on nutrient limitation of lacustrine phytoplankton has focused on phosphorus (P) and nitrogen (N) limitation, there is growing evidence that iron (Fe) limitation may be more common than previously acknowledged. Here we show that P was the nutrient that stimulated phytoplankton primary production most strongly in seven out of nine bioassay experiments with natural lake water from oligotrophic clearwater lakes. However, Fe put constraints on phytoplankton production in eight lakes. In one of these lakes, Fe was the nutrient that stimulated primary production most, and concurrent P and Fe limitation was observed in seven lakes. The effect of Fe addition increased with decreasing lake water concentrations of total phosphorus and dissolved organic matter. Possible mechanisms are low import rates and low bioavailability of Fe in the absence of organic chelators. The experimental results were used to predict the relative strength of Fe, N, and P limitation in 659 oligotrophic clearwater lakes (with total phosphorus £ 0.2 lM P and total organic carbon < 6 mg C l )1 ) from a national lake survey. Fe was predicted to have a positive effect in 88% of these lakes, and to be the nutrient with the strongest effect in 30% of the lakes. In conclusion, Fe, along with P and N, is an important factor constraining primary production in oligotrophic clearwater lakes, which is a common lake-type throughout the northern biomes.

The Absorption of Light in Lakes: Negative Impact of Dissolved Organic Carbon on Primary Productivity

“…Because CDOM and DOC are so closely correlated with Fe, the negative effect of CDOM or DOC possibly accounts for most of the Fe effect so that little residual variation can be explained by including Fe. Furthermore, Fe may stimulate PP in low DOC-lakes in the same region (Vrede and Tranvik 2006). Whether this may cancel out the light-absorptive effects remains unsettled, but is worth pursuing.…”

Responses in zooplankton populations to food quality and quantity changes after whole lake nutrient enrichment of an oligotrophic sub-alpine reservoir

“…The particulate N:P ratios of both Burvattnet and Mjçlk-vattnet are indicative of P-limitation of the phytoplankton community since the N:P ratios are well above the molar Redfield ratios (106C:16N:1P), especially in Burvattnet 2002 where the mean N:P was very high (Table 3). However, Vrede and Tranvik (2006) showed in nutrient limitation bioassay experiments with August 2003 water samples from Burvattnet and Mjçlkvattnet that the phytoplankton communities in both lakes were concurrently limited by P and iron (Fe), and additions of P having the largest effect in Mjçlkvattnet and Fe in Burvattnet.…”

“…The dose, which roughly corresponded to 1.5 -3 mg P L -1 , was selected after preliminary mesocosm experiments in 2001 that showed a strong response in the phytoplankton biomass to P addition and the response was saturated at P concentrations >4 mg P L -1 (Vrede et al, unpublished data). This lack of further response in algal biomass can probably be attributed to an onset of iron limitation of phytoplankton growth (Vrede and Tranvik, 2006). The N:P ratio of added nutrients was kept high to avoid N-limitation of the system that could stimulate unwanted growth of N-fixing cyanobacteria.…”

Evidence of an atmospheric forcing on bacterioplankton and phytoplankton dynamics in a high mountain lake

“…In particular, Saharan dust exhibits a significant content in Fe (Jickells et al, 2005), a micronutrient which limits productivity in wide areas of the globe, including freshwater ecosystems (Martin et al, 1994;Vrede and Tranvik, 2006). Although La Caldera Lake is mainly P-limited, potential Fe limitation has never been explored and no data on dissolved Fe concentration are available yet.…”