Peipoch, M., M. Daniels, and S.H. Ensign. 2026. Ecosphere 17(9): e70741.
Permalink/DOI (Open access)
Abstract
Phytoplankton dynamics in rivers have long challenged growth models, which predict limited biomass accumulation due to insufficient water residence times—yet phytoplankton populations often persist despite this constraint. This paradox is especially pronounced in mid-order rivers, where water residence times appear insufficient to support observed levels of phytoplankton growth.
In this study, we evaluate the role of in-channel transient storage zones—low-velocity areas such as backwaters, eddies, and side pools—in facilitating phytoplankton persistence and biomass accumulation along a fifth-order river reach. Using five years of continuous monitoring data, Bayesian inverse modeling, and storm-event sampling, we tested the storage zone hypothesis, which posits that phytoplankton growth is supported by extended residence time in these low-velocity areas.
Our findings suggest that phytoplankton growth rates in transient storage zones are, on average, 2–4 times higher than in the thalweg, and that in the fall, estimated chlorophyll concentrations in transient storage zones can be up to 4.3 times higher on average than those in the thalweg. Based on these estimated chlorophyll concentrations in transient storage zones, we infer the aggregated volume of transient storage zones along the reach is likely near 10% and not larger than 40% of the total reach volume. Model outputs of downstream chlorophyll enhancement were consistent with previous numerical models of phytoplankton persistence and empirical storm export data, particularly after low-intensity storm events.
Together, these results support the inference that biomass accumulation in transient storage zones can play a critical role in sustaining phytoplankton populations, providing up to a 65% increase in chlorophyll concentrations along the study reach.
This study represents one of the first field-based quantifications of reach-scale phytoplankton recruitment from transient storage zones and highlights the need to account for spatial and temporal heterogeneity in flow and habitat structure when modeling primary production in river networks. Phytoplankton accumulation in rivers should no longer be considered paradoxical, and storage zone size and exchange should be recognized as critical parameters affecting autotrophy in mid-order rivers.
