Managing River Flow Regimes to Suppress Cyanobacterial Growth

Planktonic cyanobacteria generally do not develop blooms in rapidly flowing rivers. Possible reason for this may be increased turbidity due to high loads of inorganic particles and hence limited light availability, losses due to benthic grazing or highly fluctuating conditions lowering growth rates (Dokulil, 1994; Reynolds et al., 1994; Welker Sc Walz, 1998; Caraco et ah, 2006) - or combinations of these and other factors that prevent cyanobacterial (and other phytoplankton) blooms from developing within the limited time water flows towards the sea, in most cases within days or weeks.

In rivers with long stretches of slow flow, like the lowland Murray- Darling River in Australia, this is different: hydrophysical conditions remain fairly constant over long stretches of such rivers. If nutrient concentrations are also high, the cyanobacteria which are typically found in well-mixed shallow waterbodies - for example, Planktothrix agardbii and other fine filamentous species - may become dominant and reach high population densities. To break their dominance, hydrophysical interventions would need to introduce pronounced changes to flow or mixing conditions at time intervals in the range of 1-2 doubling times of the cyanobacteria, that is, within several days or one to two weeks.

Impoundments or constructed barriers markedly reduce both turbulent kinetic energy and river flow and increase residence times. For example, where, without impoundment, the water would take one week to travel from the foothills to the river’s mouth (i.e., 1-2 doubling times of the cyanobacteria) impoundments can reduce this travelling time to many weeks. This gives cyanobacterial populations sufficient time for many cell divisions and thus for the formation of blooms. Where impoundments are being planned, this potential impact on water quality should be assessed. Where impoundments already exist and have been identified as one cause of cyanobacterial proliferation, managing them differently or even restoring natural flow regimes may be an option, depending on other management targets.

Low flow conditions in lowland rivers can even lead to stratification. The correlation observed between buoyant species of Dolicbospermum and low flow in some large rivers suggests that the manipulation of flow may be used to control cyanobacteria (Baker et al., 2000; Maier et al., 2004). In regulated rivers, the magnitude and timing of discharge can be manipulated to disrupt stratification every few days, thereby controlling cyanobacterial growth. Bormans and Webster (1997) developed a mixing criterion for turbid rivers that can be used to determine the flow required to disrupt stratification.

River management strategies to generate higher flow and reduce the risk of cyanobacterial blooms depend upon the availability, cost and ability to deliver enough water to provide that flow. It is also important to weigh the likelihood of success and cost-benefit of such interventions against further socioeconomic criteria (i.e., the need for an impoundment to store water or enable shipping; loss of provision of water for irrigation) and ecological criteria (i.e., the implications of flow regime changes on the riverine ecosystem that is adapted to the slow flow or impoundment regime).

Checklist 8.5 suggests questions to address when considering changes in river flow management with respect to their impact on cyanobacterial growth.

CHECKLIST 8.5: ASSESSING RIVER FLOW REGIMES AND OPTIONS FOR THEIR MANAGEMENT

  • • Are data on flow rates available? If not, can they be collected?
  • • What is the goal for the flow management or manipulation regime: to reduce residence time and dilute cyanobacteria? To disrupt stratification and reduce growth through altering mixing and light availability?
  • • What changes in flow regime are required to reach this goal?
  • • Is enough water available stored upstream in the catchment for the flow rate targeted?
  • • Which other sectors need to be involved in developing a flow management strategy?

Operational monitoring for flow regime management

Operational monitoring will record whether river flows are as planned, that is, through measuring flow rates. For major rivers, data on river flow are often available from water resource authorities who generate them for other purposes.

Validation of flow regime management

Validation of the flow regime management involves monitoring whether the intended flow rates are achieved (for measuring them, see section 7.2).

Managing Water Retention Time in Lakes and Reservoirs to Suppress Cyanobacterial Growth

Management interventions reducing water retention times in a waterbody may successfully reduce cyanobacterial biomass, if dilution rates can be achieved that are higher than their growth rates - that is, retention times of one a month or less. Retention time is the quotient of the basin volume divided by the inflow. For many waterbodies, particularly lakes, retention times are not well known, and they may be difficult to measure directly particularly if there is more than one inflow or if a lake is strongly connected to groundwater flows (see Chapter 7). Water budgets may be calculated from concentration changes of a conservative tracer substance like chloride analysed in the inflow(s), in the lake and in the outflow. As for managing river flow, a caveat may be the lack of water availability to increase the water exchange rate, particularly during seasons with little precipitation in the catchment.

CHECKLIST 8.6: ASSESSING WATER RETENTION TIMES AND OPTIONS FOR THEIR MANAGEMENT

  • • Are data on water retention times available? If not, can inflow and outflow rates be established or inferred from concentration differences of a tracer substance (like chloride)?
  • • Can a water retention time target of approximately one month or less be achieved in the lake or reservoir during the growing season?
  • • Are sufficient water volumes of suitable quality available in the catchment for this target?
  • • Are there conflicting interests for the use of this additional water or for environmental targets affected by diverting water to increase exchange rates?

Operational monitoring of water retention time management

Operational monitoring will serve to ensure that inflows to the water- body remain in the predefined range, and thus, it will require monitoring inflow or outflow to determine whether the intended retention time is actually achieved. For reservoirs, data on outflow from the dam are usually available, and data on drinking-water abstraction may need to be included in the budget, if this volume amounts to more than a few percent of the river flow.

Validation of water retention time management

Validation of the water retention time will focus on checking whether it is indeed short enough in all parts of the waterbody to achieve its target of reducing cyanobacterial biomass. Particularly for waterbodies with many bays, as is typical for reservoirs, retention time may not be homogenous and inflows may find a preferential flow path through them, with much lower water exchange rates in the bays. Monitoring cyanobacterial occurrence also at such locations is particularly important so that further measures can be taken if the management of retention time proves insufficient.

 
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