Recent global climate changes have altered the land utilisation type of flood pains; furthermore, the influence of human activities on water discharge regulation has caused extensive changes to the hydrological characteristics of watercourses. Moreover, the original mobility, connectivity, and hydrodynamic characteristics of watersheds have further changed along with the development of freshwater resources and the construction of dam gates. With the acceleration of dam gate construction in watersheds, natural river-type water bodies turn into lake-type water bodies upstream of gates, and the hydrodynamic gentleness of watersheds caused by the simultaneous operation of dam gates has

Abstract. We developed an approach to assess environmental flows in estuaries related to preference of phytoplankton considering the complex relationship between hydrological modification and biomass in ecosystems. As a first step, a relationship was established between biomass requirements for organisms of primary and higher nutritional levels based on the principle of nutritional energy flow of ecosystem. Then, diagnostic pigments were employed to represent phytoplankton community biomass, which indicated competition between two groups of phytoplankton in the biochemistry process. Considering empirical relationships between diagnostic pigments and critical environmental factors, responses of biomass to river discharges were established based on a convection–diffusion model by simulating distributions of critical environmental factors under action of river discharges and tide currents. Consequently, environmental flows could be recommended for different requirements of fish biomass. In the case study in the Yellow River estuary, May and October were identified as critical months for fish reproduction and growth during dry years. Artificial hydrological regulation strategies should carefully consider the temporal variations of natural flow regime, especially for a high-amplitude flood pulse, which may cause negative effects on phytoplankton groups and higher organism biomass.

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Pulse flow, which includes base flow, peak flow, flow duration, occurrence time, and frequency, is a manifestation of environmental flow in rivers. This paper describes a methodological framework for determining pulse flow based on an analysis of fish spawning behavior and presents its application to the second Songhua River in northeastern China. Peak flow was determined based on the hydrographic-habitat relationship for fish spawning in conjunction with physical habitat simulation. The flow duration was determined based on the incubation period of fish eggs. The occurrence time and frequency were determined based on the suitable temperature for fish spawning. This application differs from conventional methods for dividing fish spawning periods and selecting target species in the corresponding period. Fish were divided into cold-water fish, hypothermal-water fish, and warm-water fish according to spawning temperature, and the target species in each month of the spawning period (April to July) were identified. For the same spawning period, the hydrographic-habitat relationships of target species with adhesive and drifting eggs were weighted to determine the peak flow. The most suitable peak flow for fish spawning from April to May in the research region is 900 m3/s and 1200 m3/s for June to July. Using the proposed framework, an ecological hydrograph from April to July was obtained by combining the method of pulse flow with habitat simulation. Fifteen days, eight days, and five days were chosen as the flow durations for April, May, and June to July, respectively. We recommend scheduling two high pulse flows each month from May to July while limiting the occurrence to only one in April. The results show that this framework offers a promising approach for developing environmental flows in rivers with a variety of fish species.