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Limnol. Oceanogr., 52(2), 2007, 649–661 E 2007, by the American Society of Limnology and Oceanography, Inc. Accounting for grazing dynamics in nitrogen-phytoplankton-zooplankton models Aditee Mitra and Kevin J. Flynn1 Institute of Environmental Sustainability, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, United Kingdom Michael J. R. Fasham National Oceanography Centre, Southampton SO14 1ZH, United Kingdom Abstract Nitrogen-phytoplankton-zooplankton (NPZ)–type models are widely used to explore the dynamics of marine planktonic ecosystems. Within these models, grazing by zooplankton on phytoplankton that are subjected to varying degrees of nitrogen limitation is described using N-based grazing kinetics together with fixed N assimilation efficiency. There is no empirical evidence for zooplankton displaying such behavior; on the contrary, there is evidence for a decline in zooplankton growth rates on consumption of N-impoverished prey, with decreased assimilation efficiencies coupled with decreased ingestion rates of nutrient-limited (i.e., poor quality, low N : C) prey. Unwittingly, then, traditional NPZ models make unjustified assumptions concerning changes in predator–prey interactions on consumption of low-quality prey. We explore the effects of this flawed description, also asking why NPZ models can still give reasonable descriptions of field data. Our conclusion is that one flaw may be countered by another, namely, by an inadequate description of nongrazing phytoplankton losses. In nature, these nongrazing losses are enhanced within nutrient-depleted phytoplankton populations. In models, a decline in grazing losses on nutrient-deprived phytoplankton is, de facto, compensated for by enhanced nongrazing losses. While the fit of the revised model to the data is not dissimilar to that of the original model (with its inappropriate descriptions of grazing and nongrazing phytoplankton mortality), the fate of primary production is very different. With the biologically more acceptable description, more material flows through the detrital compartment, with important implications for trophic dynamics. Care must be taken not to oversimplify descriptions of biology in models, as these may directly or indirectly mask the simulation of other important ecological processes. Over the past decade there has been a concerted effort to increase the realism of ecosystem models that describe plankton production. Most of this effort has been expended on the description of phytoplankton; thus, multinutrient, photoacclimation models are now not uncommon (e.g., Fasham et al. 2006). Zooplankton grazing functions have been shown to be important determinants for plankton system dynamics (Steele 1976; Steele and Henderson 1992; Strom and Loukos 1998; Mitra and Flynn 2006). Thus, it is just as important to justify the construction of zooplankton models for incorporation within ecosystem models as it is to do so for phytoplankton models (Flynn 2003, 2005b). However, the description of zooplankton models has changed little, other than by the inclusion of more zooplankton groups (micro-, mesozooplankton and so on; e.g., Blackford et al. 2004) and by the greater appreciation of the importance of stoichiometry (e.g., carbon : nitrogen : phosphorus [C : N : P) in affecting assimilation and hence gross growth efficiency of the zooplankton predator (e.g., Sterner and Elser 2002; Anderson et al. 2005). These zooplankton models do not consider the effects of prey quality and availability on prey selection, ingestion kinetics, and growth dynamics. 1 Corresponding author (k.j.flynn@swansea.ac.uk). Acknowledgments This work was supported by the Natural Environment Research Council (United Kingdom). We gratefully acknowledge review comments on previous versions of this work. Recently, we have developed models to enable a consideration of the effects of prey of different quality and quantity on zooplankton ingestion and growth kinetics (Mitra 2006; Mitra and Flynn 2006a, 2006b). These factors are important because changes in phytoplankton nutrient status, typically associated with nutrient exhaustion and often with the accumulation of toxins (Cembella 2003), can stimulate changes in zooplankton predation behavior (Irigoien et al. 2005; Mitra and Flynn 2005). Such changes in behavior, associated with stoichiometric disparity between predator and prey, have been termed stoichiometric modulation of predation (SMP; Mitra and Flynn 2005). In this paper we consider the implications of SMP for the performance of what can probably be considered the classic modeling description of marine planktonic ecosystems, namely, the nitrogen-phytoplankton-zooplankton (NPZ) model (Fasham et al. 1990; Evans and Garc¸on 1997; Franks 2002). Predation operates at the level of the particle; individual food items are selected, captured, and ingested. However, most planktonic ecosystem models do not use the individual as a state variable but use biomass instead; NPZ models describe biomass in terms of nitrogen. Generally, carbon biomass correlates well with the number of individuals for a given type of organism. Phytoplankton carbon per cell (C cell21) is more constant for a given species than is their nitrogen content during N deprivation (e.g., Davidson et al. 1992). Phytoplankton N : C varies with the availability of nutrient N over a three- to fivefold range, depending on the phytoplankton type (e.g., Flynn et 649 650 Mitra et al. Fig. 1. Effects of prey quality on predator growth. (A) Neutral stoichiometric modulation of predation (SMP) for ingestion and assimilation (0 SMPIng and 0 SMPAE, respectively), with no modification of the ingestion behavior or assimilation efficiency for the limiting nutrient (nitrogen; AEN) in response to the presence of poor-quality prey. (B) +ve SMPIng with 0 SMPAE; ingestion of prey C is increased, so maintaining N ingestion. (C) 0 SMPIng with +ve SMPAE; AEN is enhanced to a maximum of 100%. (D) 2ve SMPIng with 0 SMPAE; C ingestion decreases, decreasing further N ingestion. (E) 0 SMPIng with 2ve SMPAE; as panel (A) but with AEN decreasing. (F) +ve SMPIng with 2ve SMPAE; as panel (B) but with AEN decreasing. The original configuration of NPZ-type models for phytoplankton predation accords with the pattern in panel (B). Y-axis units are indicative of relative change with food quality only. al. 1993; John and Flynn 2002). Transformations from cell volume are used to yield carbon rather than nitrogen biomass (e.g., Postel et al. 2000). Let us, therefore, assume that C cell21 indeed remains constant regardless of N : C in phytoplankton. The default, simplest expectation is that as phytoplankton N : C declines during nutrient exhaustion, there is no change in cell-based and hence C-based capture kinetics. Likewise, the simplest expectation is that assimilation efficiency for the limiting nutrient (in this instance AE for N [AEN]) is unaffected. This behavior, with no response by the predator to the consumption of poorquality (low N : C) prey, is neutral stoichiometric modulation of predation (0 SMP; Mitra and Flynn 2005). The implications of 0 SMP are shown in Fig. 1A; if predation rate in terms of cell number and hence C ingestion is unaffected by prey N : C and all other factors (such as AEN) remain constant, then there is a decline in predator growth rate pro rata with the decline in N ingestion. Most zooplankton models make such assumptions in their construct or exhibit no functional relationship between ingestion and prey quality (e.g., Sterner and Elser 2002; Anderson et al. 2005; cf. Mitra 2006). When confronted with prey of low N : C, predators may be expected to modify their behavior in order to counter the decline in prey quality; this is termed positive SMP (+ve SMP; Mitra and Flynn 2005). This could be accomplished by enhancing the ingestion rate and/or by enhancing the assimilation efficiency of what is ingested. These behavioral responses are termed + 内容过长，仅展示头部和尾部部分文字预览，全文请查看图片预览。 try. Oikos 84: 537–542. TIRELLI, V., AND P. MAYZAUD. 2005. Relationship between functional response and gut transit time in the calanoid copepod Acartia clausi: Role of food quantity and quality. J. Plankton Res. 27: 557–568. TSEITLIN, V. B. 1999. The influence of faecal pellets sinking rate and the distribution of planktonic animals on organic carbon flux from the upper layer of the ocean. Okeanologiya 39: 248–252. Received: 19 December 2005 Accepted: 6 September 2006 Amended: 2 October 2006 [文章尾部最后500字内容到此结束,中间部分内容请查看底下的图片预览]请点击下方选择您需要的文档下载。

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