Environment B·O·P Environmental Report 2000/06 March 2000 Prepared by T K Wilding, Environmental Scientist Environment B·O·P Quay Street P O Box 364 Whakatane NEW ZEALAND ISSN 1172 - 5850 Acknowledgements Rob Donald oversaw the lake algae monitoring till 1997 and his earlier reports provided valuable guidance. John McIntosh co- authored the foam chapter and provided valuable advice for the other chapters. Frank Thompson and the late Maureen Doherty collected valuable data on the Rotorua foam as well as feedback on other lake issues. Nick Carter, Daimen Jones, Mark Hammer and Alex Sides collected and analysed samples and assisted in data analysis. John Gibbons-Davies undertook thelake sampling and produced maps and diagrams for this report, as did Gareth Evans. Dr Rick Pridmore (NIWA) peer reviewed the draft and his comments were most useful. Comments on the draft were also gratefully received from John McIntosh, Rob Donald, Dr Jo Deeley, Rob Pitkethley (Eastern Fish & Game), Ross Price (Pacific Health) and Paul Dell. Cover Photo: Lake Rotorua Foam 1998 (Courtesy M Doherty), inset Kirchneriella lunaris (under microscope). Lake Rotoma, inset clean water algae. Lake Rotorua blue green bloom 1997 (R Donald), inset Anabaena spiroides (R Donald). Contents Executive Summary............................................................................. i Chapter 1: Preface .............................................................................. 1 Chapter 2: Rotorua Lakes Algal Monitoring 1991-1998................. 3 2.1 Introduction ................................................................................................ 3 2.2 Methods .................................................................................................... 4 2.3 Results and Discussion ............................................................................. 8 2.4 General Patterns ..................................................................................... 11 2.5 Multivariate Analysis ................................................................................ 11 2.6 Phytoplankton as Environmental Indicators .............................................. 12 2.7 Conclusions ............................................................................................ 14 Chapter 3: Rotorua Lakes Blue-Green Algae Monitoring ............33 3.1 Introduction .............................................................................................. 33 3.2 Methods .................................................................................................. 35 3.3 Results .................................................................................................... 36 3.4 Discussion .............................................................................................. 45 3.5 Conclusions ............................................................................................ 51 Chapter 4: Lake Rotorua Foam ........................................................53 4.1 Introduction .............................................................................................. 53 4.2 Methods .................................................................................................. 55 4.3 Results and Discussion ........................................................................... 56 4.4 Conclusions ............................................................................................ 62 Chapter 5: General Discussion........................................................63 5.1 Trends ..................................................................................................... 63 5.2 Causes ................................................................................................... 63 5.3 Predictions.............................................................................................. 64 5.4 Consequences ........................................................................................ 65 5.5 Monitoring ............................................................................................... 65 References ..........................................................................................67 Appendices .........................................................................................73 Glossary ..............................................................................................97 i Executive Summary Results are presented for phytoplankton monitoring of the Rotorua Lakes since 1991. The general phytoplankton community was monitored in conjunction with water chemistry sampling. In addition, shore monitoring of blue green algae has provided a more detailed understanding of a specific nuisance algae in three of the lakes. Major findings are summarised in Table 1. The trophic status describes the productivity of the lake, eutrophic lakes being more enriched than oligotrophic lakes. Nuisance algae blooms include blue green algae, which produce toxins, and Kirchneriella which is associated with foam production. These impact on the natural and economic values of a lake. Long term trends in trophic state, as presented in the table, reflect changes in various phytoplankton parameters (e.g. chlorophyll-a, blue green concentrations, clarity, etc.). The water quality of Lake Rotoehu has declined since 1992 with riparian planting undertaken in response. The few years’ data we have for Okawa Bay suggest blue green algae have become more persistent. Rerewhakaaitu was improving up until the time of the last lakes report (1995), but has declined sharply since, in spite of extensive lake and stream margin retirement. Rotorua has made a slight improvement, but this was somewhat overwhelmed by isolated blooms of nuisance algae. Monitoring increased our understanding of blue green algae and allowed predictions to be made about the long term risk of other lakes developing such blooms. Calm and shallow high nutrient conditions are favoured and hence Rerewhakaaitu is considered to be a high-risk lake. The tendency of soils in this catchment to mop up phosphorous appears to have mitigated the effects of widespread agriculture to date. Because Lake Rotorua is so exposed, wind speeds are expected to be limiting blue green blooms more so than nutrients. The western end of Lake Rotoiti is shallow and receives nutrient rich water from Rotorua. Flushing is expected to limit the establishment of blue greens through competition with algae already dominant in the feeder waters. Some lakes are unusual in so much as their algae communities seem to deviate from the expected composition. The chemical or physical features that cause this are listed in the last column of Table 1. For example, low reactive silica concentrations (and possibly other major ions) of Lake Tikitapu inhibit the growth of diatoms resulting in a lake completely dominated by green algae. During winter of 1998, Lake Rotorua was plagued by a sticky white carbohydrate foam. This was caused by a ‘new’ species of algae that became dominant in the lake (Kirchneriella lunaris). A combination of unusual climatic conditions and the dynamic nature of this lake appear to have favoured the alga at this time. There have been no recurrences since. Mouse bioassays of blue green blooms have infrequently found toxins present. Although few samples have been analysed, it does highlight the need for this issue to be addressed in greater detail. Phytoplankton monitoring provides valuable information on the consequences of eutrophication and how best to manage resources in the lake environment. New methods were developed to simplify the interpretation of algae data in terms of trophic conditions and provide a standard approach for trend monitoring (phytoplankton community indices). Environmental Report 2000/06 Rotorua Lakes Algae Report ii Environment B·O·P Rotorua Lakes Algae Report Environmental Report 2000/06 1 Chapter 1: Preface Environmental monitoring is a specific requirement of the Resource Management Act, 1991 (Part IV, section 35, 1 and 2a). Integrated management of our natural and physical resources requires a holistic approach to data gathering. To this end, the Natural Environment Regional Monitoring Network investigates the important physical, chemical, and biological characteristics of the Bay of Plenty region. Such information allows managers to assess the present and potential impacts of consent related activities and provides baseline environmental data. Additionally, long term trend detection using environmental baseline data will enable Council to assess the effectiveness of its own policies and plans in maintaining or enhancing the environment and in meeting its statutory obligations under the Resource Management Act. This report presents the results of two monitoring programmes and one environmental investigation, presented in separate chapters. All relate to lake phytoplankton, which are microscopic plants drifting freely in open water. Chapter 2 presents the results of the general algal monitoring carried out in conjunction with the water quality monitoring. Whereas these samples are collected from a boat using a 10m tube, the monitoring described in Chapter 3 relies on shore samples. Chapter 2 looks at the whole community while Chapter 3 focuses on one nuisance taxa – blue green algae (Cyanobacteria). Chapter 4 employs the data from both programs to investigate a foam problem that affected Lake Rotorua during 1998. Additionally this chapter presents results of specialised chemical analysis undertaken