3.0 Methods to Control Nutrients 3.1 Introduction

Generic Environmental Impact Report for Eutrophication and Aquatic Plant Management in Massachusetts 3.0 METHODS TO CONTROL NUTRIENTS 3.1 INTRODUCTION One of the most effective ways to control algal populations is by limiting the nutrient supply to the lake, and thus limiting growth of algae. This approach may work with some rooted aquatic plants as well, but as most rooted plants acquire most of their nutrition from the sediment (Barko and Smart, 1981), control of nutrients in the water column is far more effective as an algal management strategy. As previously discussed in Section 1, phosphorus is the best nutrient to control, and the discussion of this section will deal primarily with phosphorus control methods. In nutrient rich lakes, the growth of algae may be limited by light, and reduction in nutrient concentrations may not have a significant effect until the nutrient concentrations are lowered sufficiently to induce nutrient limitation (Section 1). One must usually identify the sources of nutrients before an effective control strategy can be determined. To do this, an accurate phosphorus budget is required (Section 1.2). Once the relative importance of the sources of phosphorus is determined, one can examine the control techniques identified below for applicability and feasibility, and compare them to the “No Management Alternative” for nutrients. 3.1 Non-Point Sources – control of diffuse nutrient sources from the watershed 3.2 Point Sources – control of point sources, usually piped discharges 3.3 Hydraulic Controls – diversion, dilution, flushing, and hypolimnetic withdrawal strategies 3.4 Phosphorus Inactivation – chemical binding of phosphorus to limit availability 3.5 Artificial Circulation and Aeration – mixing and oxygen addition 3.6 Dredging – removal of nutrient-laden sediments 3.7 Additional Techniques – bacterial additives and removal of bottom feeding fish The expected reduction in phosphorus loading should be modeled as described in Section 1 to predict the change in trophic status. In general, algal problems will be minimized at loadings less than Vollenweider’s permissible level, but algal abundance in response to nutrient loading is a probability distribution, not a threshold function. Consequently, algal blooms may be expected at some reduced frequency, even at fairly low nutrient levels, and lakes will not respond identically to changes in loading. Acceptable results might be achieved at loadings higher than the permissible level, but unacceptable conditions can be expected where loading exceeds Vollenweider’s critical limit. Managers should be prepared to adjust strategies in response to resultant lake conditions; algal control through nutrient limitation is often an iterative process. Additional ways to directly limit the density of algae may be needed on an interim or supplemental basis, and include the use of biocidal chemicals, dyes or biocontrol agents. These are addressed in Section 4. A summary table of available techniques is presented in Table 3-1, adapted from Wagner (2001). Techniques that address nutrient levels are mixed with methods aimed more directly at the algae, providing a complete summary of algae control approaches, including the nutrient controls that are the subject of this section 3 METHODS TO CONTROL NUTRIENTS Page 3-1 Generic Environmental Impact Report for Eutrophication and Aquatic Plant Management in Massachusetts Table 3-1 Options for control of algae. (Adapted from Wagner 2001). OPTION MODE OF ACTION ADVANTAGES DISADVANTAGES Watershed controls 1)Management for ¨ Includes wide range of ¨ Acts against the ¨ May involve nutrient input watershed and lake original source of algal considerable lag time reduction edge activities intended nutrition before improvement to eliminate nutrient ¨ Creates sustainable observed sources or reduce limitation on algal ¨ May not be sufficient delivery to lake growth to achieve goals ¨ Essential component of ¨ May control delivery without some form of algal control strategy of other unwanted in-lake management where internal recycling pollutants to lake ¨ Reduction of overall is not the dominant ¨ Facilitates ecosystem system fertility may nutrient source, and management approach impact fisheries desired even where which considers more ¨ May cause shift in internal recycling is than just algal control nutrient ratios which important favor less desirable algae 1a) Point source ¨ More stringent ¨ Often provides major ¨ May be very controls discharge requirements input reduction expensive in terms of ¨ May involve diversion ¨ Highly efficient capital and operational ¨ May involve approach in most cases costs technological or ¨ Success easily ¨ May transfer problems operational adjustments monitored to another watershed ¨ May involve pollution ¨ Variability in results prevention plans may be high in some cases 1b) Non-point source ¨ Reduction of sources of ¨ Removes source ¨ May require purchase controls nutrients ¨ Limited or no ongoing of land or activity ¨ May involve costs ¨ May be viewed as elimination of land uses limitation of “quality or activities that release of life” nutrients ¨ Usually requires ¨ May involve alternative education and gradual product use, as with no implementation phosphate fertilizer 3 METHODS TO CONTROL NUTRIENTS Page 3-2 Generic Environmental Impact Report for Eutrophication and Aquatic Plant Management in Massachusetts Table 3 - continued Option Mode of Action ADVANTAGES DISADVANTAGES 1c) Non-point source ¨ Capture of pollutants ¨ Minimizes ¨ Does not address pollutant trapping between source and interference with land actual sources lake uses and activities ¨ May be expensive on ¨ May involve drainage ¨ Allows diffuse and necessary scale system alteration phased ¨ May require ¨ Often involves wetland implementation substantial treatments throughout watershed maintenance (detention/infiltration) ¨ Highly flexible ¨ May involve approach stormwater collection ¨ Tends to address wide and treatment as with range of pollutant point sources loads IN-LAKE PHYSICAL CONTROLS 2) Circulation and ¨ Use of water or air to ¨ Reduces surface build- ¨ May spread localized destratification keep water in motion up of algal scums impacts ¨ Intended to prevent or ¨ May disrupt growth of ¨ May lower oxygen break stratification blue-green algae levels in shallow water ¨ Generally driven by ¨ Counteraction of ¨ May promote mechanical or anoxia improves downstream impacts pneumatic force habitat for fish/invertebrates ¨ May reduce internal loading of phosphorus 3) Dilution and flushing ¨ Addition of water of ¨ Dilution reduces ¨ Diverts water from better quality can dilute nutrient concentrations other uses nutrients without altering load ¨ Flushing may wash ¨ Addition of water of ¨ Flushing minimizes desirable zooplankton similar or poorer detention; response to from lake quality flushes system pollutants may be ¨ Use of poorer quality to minimize algal build - reduced water increases loads up ¨ Possible downstream ¨ May have continuous or impacts periodic additions 4) Drawdown ¨ Lowering of water over ¨ May reduce available ¨ Possible impacts on autumn period allows nutrients or nutrient non-target resources oxidation, desiccation ratios, affecting algal ¨ Possible imp airment and compaction of biomass and of water supply sediments composition ¨ Alteration of ¨ Duration of exposure ¨ Opportunity for downstream flows and and degree of shoreline clean- winter water level dewatering of exposed up/structure repair ¨ May result in greater areas are important ¨ Flood control utility nutrient availability if ¨ Algae are affected ¨ May provide rooted flushing inadequate mainly by reduction in plant control as well available nutrients. 3 METHODS TO CONTROL NUTRIENTS Page 3-3 Generic Environmental Impact Report for Eutrophication and Aquatic Plant Management in Massachusetts Table 3 - continued Option Mode of Action ADVANTAGES DISADVANTAGES 5) Dredging ¨ Sediment is physically ¨ Can control algae if ¨ Temporarily removes removed by wet or dry internal recycling is benthic invertebrates excavation, with main nutrient source ¨ May create turbidity deposition in a ¨ Increases water depth ¨ May eliminate fish containment area for ¨ Can reduce pollutant community (complete dewatering reserves dry dredging only) ¨ Dredging can be ¨ Can reduce sediment ¨ Possible impacts from applied on a limited oxygen demand containment area basis, but is most often ¨ Can improve discharge a major restructuring of spawning habitat for ¨ Possible impacts from a severely impacted many fish species dredged material system ¨ Allows complete disposal ¨ Nutrient reserves are renovation of aquatic ¨ Interference with removed and algal ecosystem recreation or other growth can be limited uses during dredging by nutrient availability 5a) “Dry” excavation ¨ Lake drained or ¨ Tends to facilitate a ¨ Rarely truly a dry lowered to maximum very thorough effort operation; tends to be extent practical ¨ May allow drying of messy ¨ Target material dried to sediments prior to ¨ Eliminates most maximum extent removal aquatic biota unless a possible ¨ Allows use of less portion left undrained ¨ Conventional specialized equipment ¨ Eliminates lake use excavation equipment during dredging used to remove sediments 5b) “Wet” excavation ¨ Lake level may be ¨ Requires least ¨ Usually creates lowered, but sediments preparation time or extreme turbidity not substantially effort, tends to be least ¨ Normally requires exposed cost dredging intermediate ¨ Draglines, bucket approach containment

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