Why are diverse relationships observed between phytoplankton biomass and transport time?
نویسندگان
چکیده
Transport time scales such as flushing time and residence time are often used to explain variability in phytoplankton biomass. In many cases, empirical data are consistent with a positive phytoplankton–transport time relationship (i.e., phytoplankton biomass increases as transport time increases). However, negative relationships, varying relationships, or no significant relationship may also be observed. We present a simple conceptual model, in both mathematical and graphical form, to help explain why phytoplankton may have a range of relationships with transport time, and we apply it to several real systems. The phytoplankton growth–loss balance determines whether phytoplankton biomass increases with, decreases with, or is insensitive to transport time. If algal growth is faster than loss (e.g., grazing, sedimentation), then phytoplankton biomass increases with increasing transport time. If loss is faster than growth, phytoplankton biomass decreases with increasing transport time. If growth and loss are approximately balanced, then phytoplankton biomass is relatively insensitive to transport time. In analyses of several systems, portions of an individual system, or time periods, apparent insensitivity of phytoplankton biomass to changes in transport time could arise due to the superposition of cases with different phytoplankton–transport time relationships. Thus, in order to understand or predict responses of phytoplankton biomass to changes in transport time, the relative rates of algal growth and loss must be known. Aquatic scientists and resource managers commonly invoke time for transport through a surface water body to help explain variability in phytoplankton biomass, often seeking empirical relationships between phytoplankton and transport time scales such as flushing time and residence time to characterize that variability. A positive phytoplankton–transport time (P–T) relationship suggests that as transport time increases (or decreases), so does phytoplankton biomass or production. Observations consistent with a positive P–T relationship are frequently made in rivers and lakes (Søballe and Kimmel 1987; Reynolds 2000; Allan and Benke 2005), floodplains (Schemel et al. 2004; Ahearn et al. 2006), estuaries (Howarth et al. 2000; Jassby 2008), and lagoons (Torréton et al. 2007), and weak flushing (long transport time) has been identified as a condition favoring harmful algal blooms in aquatic systems across the globe (Paerl and Huisman 2008). High flow (short transport time) has thus been offered as an explanation for low phytoplankton biomass and resistance to eutrophication (Wetzel 2001; Caraco et al. 2006). Despite the prevalence of positive P–T relationships in nature, negative (Søballe and Bachmann 1984), spatially variable (Søballe and Bachmann 1984; Paerl et al. 2006), temporally variable (Alpine and Cloern 1992; Strayer et al. 2008), or non-monotonic (Walz and Welker 1998; Hein 1 Corresponding author ([email protected]).
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