Análisis de rugosidad de las placas

Nutrients from past activities and land uses are dispersed in the bottom sediments of both Lakes Rotorua and Rotoiti (internal loading) and can be released into the lake water. Nutrients also enter the lakes from land use. The current inputs are the ones entering the lake now and exports will enter the lake in the future from current land use (nutrient inputs detailed in Appendix L). Nitrogen exports from land use are higher than inputs for Lake Rotorua due to the time it takes nitrogen to pass through the land and groundwater system to the lake (lag time). There is no groundwater lag time in the Rotoiti catchment, hence nutrient inputs are not expected to increase over time. While nitrogen leaches through the land, most of the sediment and phosphorus is transported to freshwaters during short periods after heavy rainfall; for example, half of the phosphorus and sediment loading from two major stream inflows to Lake Rotorua occurred about 15% and 1% of the time, respectively (Hamilton et al., 2013). Nutrient input targets have been established to improve water quality in the lakes (targets in Appendix L) (Environment Bay of Plenty et al., 2009).

Lake Rotoiti is connected to Lake Rotorua by the Ohau Channel where almost 70% of the nitrogen and 86% of the phosphorus entered Lake Rotoiti from Lake Rotorua (Environment Bay of Plenty et al., 2009). Hence, water quality in Lake Rotorua previously affected Lake Rotoiti. This was before the Ohau Channel wall was completed in 2008. The wall was built to prevent nutrient rich water flowing to Lake Rotoiti. Instead nutrients have been diverted down the Kaituna River (Bay of Plenty Regional Council et al., 2010). Accordingly, inputs into Rotoiti are

Valuation of Impacts 139 expected to drop considerably when nutrient reductions from the wall are included. The

diversion wall cost around $10 million (Bay of Plenty Regional Council et al., 2010), mostly funded by Bay of Plenty Regional Council, with additional finances coming from central Government (Abell et al., 2011) (i.e. from ratepayers and taxpayers). The short and long term effects of the diversion on Lake Rotoiti and the Kaituna River are uncertain (Parliamentary Commissioner for the Environment, 2006). Nutrient problems in Lake Rotoiti may be remediated, but consequently shifted downstream (Ford-Robertson, 2013a).

Lake Okaro is the smallest of the publically managed Rotorua lakes and its trophic state is supertrophic (very high nutrients and algal productivity) (Bay of Plenty Regional Council et al., 2010). Pastoral farming comprises 90% of the catchment with 10% in dairy farming (38 ha), although about 20% of the nutrient load into Lake Okaro is from dairying (Figure 9.2) (Environment Bay of Plenty et al., 2006). There is less infiltration to groundwater in the Lake Okaro catchment as it has a small, elevated groundwater catchment so a long lag time is unlikely. This means that the lake water quality generally reflects the current catchment land use (Environment Bay of Plenty et al., 2006).

Lake Rotoehu is a shallow lake (average depth is 8.2 m). Dairy farming covers about 5% of the catchment but contributes to 25% of the nitrogen load and 7.5% of the phosphorus load into the lake (Figure 9.2). Algal production in the lake has been a problem as early as the 1960s. In the 1990s nutrient levels in the lake doubled from the 1970s and algal mass in the lake quadrupled (Bay of Plenty Regional Council et al., 2007). Subsequently, from 1993/94 cyanobacteria blooms have occurred in the lake every summer except one (Bay of Plenty Regional Council et al., 2007). Increased nutrients have contributed to de-oxygenation of bottom waters, triggering nutrient releases from sediments in the lakebed (Bay of Plenty Regional Council et al., 2007).

Nutrient loads in catchments

In all four of aforementioned lake catchments dairy farming contributes a greater amount of nitrogen loading than its proportional land area, with the greatest impact in Lake Rotorua (38% of total N from 11% of the catchment) (Figure 9.2). Phosphorus loading from dairy farming is relatively similar to the proportion of land in dairy farming, except for Lake Okaro which contributes almost double the proportion of P than its land area. Nutrient inputs from dairy and some other land uses are shown in Appendix M.

Valuation of Impacts 140 Figure 9.2: The proportion of land in dairy farming and proportion of nutrient loads (N and P) from

dairy land in four Rotorua Lake catchments. Data source: Environment Bay of Plenty et al. (2007).

Limiting nutrient inputs in the Lakes

The Rotorua Lakes Protection and Restoration Action Programme was established to identify issues and propose actions to address water quality problems. The Government has committed $72.1 million towards the programme which Environment Bay of Plenty (EBOP) and Rotorua District Council (RDC) will match ($144.2 million in total). Four Rotorua lakes will receive this funding: Lakes Rotorua, Rotoiti, Rotoehu and Okareka (Environment Bay of Plenty et al., 2009).

RDC proposes to spend $98 million over 10 years on new community sewerage schemes and other initiates that will benefit the lakes, including water conservation strategies and engineering expertise (Parliamentary Commissioner for the Environment, 2006). Waikato University has been awarded $10 million to research lake restoration over 10 years, focusing on the Rotorua Lakes (Parliamentary Commissioner for the Environment, 2006). The costs of the Proposed Action Plan for Lakes Rotorua and Rotoiti are expected to be around $10 million per year, excluding reductions from land use and management changes. This would achieve a reduction of around 59 tonnes N and 16 tonnes P per year (Kerr & Lock, 2009), costing about $169,500 per tonne of N ($170 per kg N).

0 5 10 15 20 25 30 35 40

Rotorua Rotoiti Okaro Rotoehu

Perc

entage

Lake catchment

Valuation of Impacts 141 9.1.3.2.Mitigation actions

A range of actions have been implemented to reduce nutrient loads from the internal (lake bed sediments) and external (land-use) sources in the lakes. Lake-based actions are reactive or ‘end of pipe’ techniques because they aim to reduce nutrient loads once nutrients have already entered streams or lakes, rather than reducing nutrients at the source. Although these actions are required for already degraded ecosystems, experience shows that techniques designed to reduce internal loads, without addressing external loads, often have limited success with short-lived benefits or delayed improvements (Abell et al., 2011). Consequently, repeated actions may be needed and failures can frequently occur, potentially causing additional environmental impacts, not to mention the high economic cost often involved for these actions. A summary of some lake-based actions and costs that have been investigated or carried out in the Rotorua Lakes are listed in Appendix N. More detailed mitigation costs for some actions are presented in Table 9.2. Options related to the Rotorua Lakes are expanded upon here, while more general examples of land management (i.e. riparian zones) are discussed in section 10.8 of Chapter 10.

Table 9.2: Actions carried out in the Rotorua Lakes and associated costs Action

Nutrient reduction

kg/year Cost $ Cost $/kg nutrients

N P N P Wetlands - Constructed – Rotoehu 3.2 ha 1650 (516/ha) $1 million $125,000/year $76/year - Constructed - Okaro 348 16 $520,000 $1494 ($176/year) - Floating – Rotoehu (2800 m2) 220 - 340 $690,000 $2,029 - $3,136 - Floating – Proposed Rotorua 220 - 3650 2-12 $900,000 $246 - $4,090 Riparian

- Okaro – fencing, planting,

restoration 423 37 $200,000

1

$473 $5,405 - Rotoehu – protection and

environmental programmes 542 249 $100,513 $185 $404

Weed harvesting – hornwort

(Rotoehu) 2,400 320 $52,800 $22 $165

Sediment capping / phosphorus inactivation

- Okaro – Phosphorus

absorbent lakebed cap 240 380

$225,000 over 3

years (lasts 7 years) $134 $84.6

- Rotorua - Alum dosing $1 million

- Okareka - phoslock $300,000 capital

- P flocculation – in 3 streams

Lake Rotorua 0 6,000 $1.260 million/year $210

Ohau channel diversion wall 150,000 15,000 $10 million

Valuation of Impacts 142 Data source: Environment Bay of Plenty et al. (2006); Bay of Plenty Regional Council et al. (2007);

Environment Bay of Plenty et al. (2009). Notes: 1Does not include all the costs; costs listed in Environment Programmes are given on a per-property basis (Environment Bay of Plenty et al., 2006). Differing cost estimates were available; hence some costs are available per year whereas others are per project.

Nutrient reduction using wetlands

Wetlands can remove dissolved nitrogen, sediment and toxins from water flows (Environment Bay of Plenty et al., 2006, 2007; Hamill et al., 2010). The process relies on the build-up of organic rich sediments and abundant denitrifying bacteria. Wetlands will take up high amounts of nutrients to start with, but over time if the plants are not harvested, nutrient uptake will cease (Hamill et al., 2010). Therefore, wetlands require maintenance and renewal over time. Wetlands remove some small amounts of phosphorus. For example, five Waikato wetlands receiving treated effluent were assimilating between 2% and 14% of the total phosphorus (Environment Bay of Plenty et al., 2006). However, eventually wetlands will build- up phosphorus and may end up releasing it; therefore, phosphorus removal is required (Environment Bay of Plenty et al., 2006). Floating wetlands remove internal lake stores of nutrients but do not target nutrient outputs from the land. Thus, they are only useful if nutrient inputs into lakes have stopped or decreased. Wetlands also have other ecological benefits – habitat for fish, amenity values, biodiversity, aesthetic and cultural benefits. Ecosystem benefits from wetlands can be substantial; for example in the Manawatu/Wanganui region, wetland ecosystems were valued at $43,320/ha, compared with $1,831/ha for dairy land (van den Belt et al., 2009).

Cost-effectiveness has been estimated over the life of different types of wetlands (Table 9.3), accounting for on-going maintenance, renewal and lease costs. Protecting natural wetlands was estimated to be the most cost-effective, while floating wetlands were the least cost effective (Hamill et al., 2010). Despite these findings, floating wetlands have been used in several instances for restoration of the Rotorua Lakes.

Valuation of Impacts 143 Table 9.3: Average cost-effectiveness of nutrient removal for different types of wetlands

Wetland option Nutrient reduction kg/ha Cost effectiveness - $/kg (range)

N P N P

Protecting natural wetlands - - $14

(11-18) $431 (260-870) Seepage wetlands 323 2 $20 (14-29) $2,739 (1600-4720) Restoring natural surface flow

wetlands 289 10 $60 (47-85) $1,714 (1110-3190) Constructed wetlands 368 11 $79 (64-97) $2,548 (1650-4600) Floating wetlands 714 13 $437 (330-570) $24,271 (17000- 35900)

Data source: Hamill et al. (2010). Cost effectiveness = amount of nutrients removed for monetary cost.

Measures to target phosphorus

A phosphorus cap is an in-lake absorbent cap that will absorb the phosphorus released from lakebed sediments, thus limiting the release of phosphorus into lakes. In-lake actions like lakebed capping speed up the restoration process by decreasing algal biomass and improving lake water quality (Environment Bay of Plenty et al., 2006). A lakebed cap was proposed to cover the 20 ha of Lake Okaro that is deep enough to turn anoxic, with a total cost of $225,000 ($75,000/year) (Environment Bay of Plenty et al., 2006).

Alum was also applied to Lake Okaro in 2003 to remove dissolved phosphorus in the lake. It binds to phosphorus in the water column and settles on the lakebed. The alum mixed throughout the surface waters within five days and lowered phosphorus concentrations by 20% (Environment Bay of Plenty et al., 2006). However, the effectiveness of the Alum application in Lake Okaro appeared to be only temporary and phosphorus concentrations increased in 2006-2007. This could be due to the low Alum concentrations or the pH in surface waters exceeding 9 as pH outside the recommended range of 6-8 can significantly reduce the P sorption capacity of Alum (Ozkundakci et al., 2010).