The Light: Nutrient Ratio in Lakes: The Balance of Energy and Materials Affects Ecosystem Structure and Process

Abstract

The amounts of solar energy and materials are two of the chief factors determining ecosystem structure and process. Here, we examine the relative balance of light and phosphorus in a set of freshwater pelagic ecosystems. We calculated a ratio of light: phosphorus by putting mixed-layer mean light in the numerator and total P concentration in the denominator. This light: phosphorus ratio was a good predictor of the C:P ratio of particulate matter (seston), with a positive correlation demonstrated between these two ratios. We argue that the balance between light and nutrients controls "nutrient use efficiency" at the base of the food web in lakes. Thus, when light energy is high relative to nutrient availability, the base of the food web is carbon rich and phosphorus poor. In the opposite case, where light is relatively less available compared to nutrients, the base of the food web is relatively P rich. The significance of this relationship lies in the fact that the composition of sestonic material is known to influence a large number of ecosystem processes such as secondary production, nutrient cycling, and (we hypothesize) the relative strength of microbial versus grazing processes. Using the central result of increased C:P ratio with an increased light: phosphorus ratio, we make specific predictions of how ecosystem structure and process should vary with light and nutrient balance. Among these predictions, we suggest that lake ecosystems with low light: phosphorus ratios should have several trophic levels simultaneously carbon or energy limited, while ecosystems with high light: phosphorus ratios should have several trophic levels simultaneously limited by phosphorus. Our results provide an alternative perspective to the question of what determines nutrient use efficiency in ecosystems.

Variability and control of carbon consumption, export, and accumulation in marine communities

“…Therefore, previous generalizations that macrophyte-dominated communities often exhibit higher levels of export (Duarte and Cebrian 1996;Cebrian 1999) than microalgal communities seem to be an oversimplification. Because the magnitude of autotrophic carbon exported does not differ among the community types compared here, I suggest that all these types potentially have a similar importance in fuelling allochthonous (i.e., out of the community) secondary production, although the nutritional quality of the material exported (internal nutrient concentration) needs also be considered (Sterner and Hessen 1994). The results also identify marshes as a larger organic carbon trap than the other community types, with the exception of seagrass beds and mangroves, because they accumulate a greater magnitude of refractory detritus.…”

Ecological stoichiometry of N and P in pelagic ecosystems: Comparison of lakes and oceans with emphasis on the zooplankton‐phytoplankton interaction

“…Wang et al [1996] found carbon ae between 70 and 94% depending upon phytoplankton food species). Furthermore, estimates of nutrient recycling ratios based on Sterner's (1990b) equations assume that both elements are accumulated by the consumer with equal efficiencies, which may not be a realistic assumption. Under the typical freshwater conditions of our study (seston N : P >> zooplankton N: P), much of the N ingested cannot be used if the consumer is to maintain homeostatic body N : P P accumulation efficiency (Pae) should be maximized, particularly if zooplankton are P limited, but there would be little benefit from increasing N accumulation efficiency (Nae) except under circumstances of very low Nae (e.g.…”

“…We thus test the hypothesis that differences in relative phytoplankton N and P deficiency in lakes and oceans are driven by differences in zooplankton body N : P and therefore relative rates of N and P recycling, of dominant zooplankters in marine and frcshwaters (Elser et al 1988). Alternatively, food-quality constraints, reflected in differences in seston C : N, C : P and N : P relative to zooplankton body composition, could be a key factor in driving zooplankton community composition (Sterner 1990b;Sterner et al 1992). General patterns regarding these parameters have been reported previously (Elser and Hassett 1994).…”

“…For each site the elemental imbalance and predicted recycling ratio was calculated from the seston and zooplankton elemental ratios. Elemental imbalance was calculated as seston N : P -zooplankton N : P (Sterner 1990b), so that a positive imbalance indicates seston N : P > zooplankton N : F? Recycling ratio is the N : P predicted to be recycled by zooplankton at each site given the measured zooplankton and seston N : P When N : P imbalance >O, N : Prccycletl = N : PSCblO,, -N : Pzoop,W( 1 -L), where L is the maximum accumulation efficiency of the element (Sterner 1990b).…”

“…Ecological stoichiometry traces its intellectual history to Lotka (1924) and its most explicit articulation to Reiners (1986). This approach focuses on the relative abundance (ratios) of critical elements (such as C, N, and P) during ecological interactions as means for insight into diverse phenomena such as foraging behavior of individuals, population regulation, resource competition, and nutrient limitation of primary production (Sterner et al 1992). Ecological stoichiometry may be especially appropriate for comparative ecology, because all biotic and most abiotic components of ecosystems can be characterized with respect to ratios of nutrient elements such as C, N, and P…”