Articles
Articles about our Freshwater and Estuaries-related specialist analytical services involving resource surveying and information.
On this page
- Sounding out submerged plants
- Rotorua Lakes: Lake plants speak out on lake conditions
- Submerged aquatic plants – indicators of lake trophic status?
- Macrophyte data over the internet
Sounding out submerged plants
Echo sounders used by recreational boaties can assist lake managers monitor underwater vegetation at minimal cost. Models that provide a printout or store information in a form that can be downloaded to a PC are most suitable as they provide a permanent record. Recent advances in sonar microprocessor technology, geographic information system (GIS), and differential global positioning system (D-GPS) now also provide an accurate mapping tool.
Aquatic plant management
Lake and waterways managers are often confronted with managing large areas of (potentially) surface-reaching nuisance weed beds and need to plan weed-control programmes before they become an acute problem. They need to define the location and extent of nuisance weed beds prior to implementing control measures and follow-up with monitoring to check the outcome and assess the effectiveness of weed control.
The most accurate method of gathering information is to deploy divers to record submerged vegetation data. These methods have proven to be reliable and accurate over the years but can be time consuming and costly, especially when surveying large areas. The use of echo-sounding equipment can greatly reduce the amount of SCUBA diving / snorkelling necessary by rapidly providing information such as a printout of the profile, heights of plants, bottom depth limits, and submerged vegetation distribution over large areas. The development and integration of GPS applications with sounders has made mapping and area calculations possible. Ground-truthing (by scuba, snorkelling or in clear water with a viewing box and a weighted measuring tape) is still required to identify plant species, make cover estimates and interpret echograms (Fig. 1). Echograms are a permanent record of vegetation and are an objective record (Fig. 2) of a number of vegetation attributes that can be used as a baseline for future comparisons and to validate management actions. It is far more reliable than subjective notes such as, “excellent results were achieved” (that are often made by a sub-contractor with a vested interest).
Useful features of an echo sounder
A GPS / echo sounder (with standard 200 kHz transducer) for general boat use, can be used to monitor aquatic plants. The cost of these units is about $3000.
Useful extras include D-GPS capability for more accurate positioning and NMEA (National Measurement Electronic Association) input and output, enabling direct data logging if required. Some of the key features useful for aquatic plant definition include:
- Digital storage of recorded profiles to memory cards so that images of vegetation profiles viewed on the unit can be saved and exported to a PC.
- Sensitivity, grey scale and colour can be adjusted to improve the image on the PC before printing. An echogram of a vegetated profile is shown in Fig.2.
- For mapping information such as position, depths and real time data can be stored with the digital profile data and exported as text if required. The saved information can then be analysed using a GIS application that enables spatial data plots to be drawn (Fig. 3).
- For accurate (+1 m) mapping, D-GPS can be deployed.
- The unit can be portable, allowing for difficult field applications. For example, we use a unit to record vegetation data in remote streams and rivers by sealing it in a splash-proof box (Fig. 4) that is easily deployed (even in a plastic fish bin) and pushed across a river or stream by a snorkeller (or in a canoe).
Limitations
Sonar signals primarily detect the gas content of plants so the reflected signal is stronger when more gas is present. Gas content varies between and within species of aquatic plants, so can provide variable signals along a weed bed profile, or for species lacking buoyancy return a poor signal making them difficult to define.
Surface interference or “noise” (caused by bubbles from waves or surface-reaching plants) can also affect detection. Tall dense surface-reaching weed beds often make it difficult to determine where the lakebed is, as dense weed beds with a strong signal obscure the return echo (Fig. 5). Reducing sensitivity (a menu function) can improve definition of the lake floor, whereas higher sensitivity settings will show less buoyant vegetation and vegetation further from the surface and allow profiles to be recorded at greater boat speed. Increasing sensitivity can also make weed beds appear taller and obscure the lake floor, providing double images. For this reason it is necessary to ground-truth the water depth and weed bed height (with a shot line or diver observations) to ensure settings are appropriate for each vegetation type. Shallow water (<2 m) usually requires different settings from deep water (2–10 m deep).
It is difficult or impossible to use an echogram to identify species or determine vegetation cover or density, so that conventional methods (e.g. SCUBA diver) are still required to ensure meaningful interpretation. Within these limitations the echograms remain useful objective records of lake vegetation as demonstrated in Fig. 1 particularly when supported by SCUBA or snorkel observations.
Rotorua Lakes: Lake plants speak out on lake conditions
Aquatic plants are valuable indicators of lake health. They are easy to measure and they integrate long-term climatic and environmental influences. LakeSPI (Lake Submerged Plant Indicators) is a new tool that uses information based on aquatic plant measurements taken from within lakes in order to generate a score indicating lake condition. LakeSPI creates a native and an invasive plant condition index and it is these scores that are integrated to create an overall lake condition index. The LakeSPI method can also be applied to historical lake vegetation survey data to generate a score reflecting former condition, which can then be compared to present day vegetation status.
Read 'Lake plants speak out on lake conditions' (PDF 3.3 MB)
Five Rotorua lakes (Rotorua, Rotoiti, Rotoehu, Okareka and Okaro) were selected at the request of Environment Bay of Plenty to carry out a LakeSPI analysis. To do this we applied our LakeSPI method to historical full lake vegetation surveys carried out on these five lakes in the 1980s. Next we selected five sites within each of these five lakes, based on historical records, and then we resurveyed these sites in October 2003 using the LakeSPI method.
LakeSPI results highlight that while lakes Rotoiti and Okareka have continued to decline in lake condition, lakes Rotoehu and Okaro have remained in a stable yet degraded state since the 1980s. On the other hand, Lake Rotorua has shown some small improvement, largely on account of its high exposure that helps to minimise weed impact and can enhance water quality features. Earlier findings showed that all five lakes underwent a major decline from their pre-invasive condition in the early 1900s; however their decline since the 1980s has been minimal in comparison. This is largely because invasive weed species have had a major impact during the 1960s to 1980s, while water quality deterioration has been a long-term and progressive influence. Despite this, it was clear that all five lakes are now dominated by invasive weed species, native vegetation has been largely displaced, former deepwater charophyte communities (as can still be found in Lake Rotoma) have all but disappeared, and the health of remaining aquatic vegetation is clearly compromised by smothering growths of filamentous blue-green algae. These benthic and epiphytic growths are a natural corollary of advancing eutrophication and provide the subsurface equivalence to surface blue-green algal blooms being experienced with increasing frequency in these lakes. Such growths are a harbinger of total macrophyte collapse as has been experienced in many Waikato lakes.
Despite the Rotorua lakes showing varying degrees of degradation they have the potential to get much worse! Protection of these lakes is far more feasible and cost effective than attempting to restore them once they become excessively degraded or devegatated. Many of the Rotorua lakes still have significant native plant communities and biodiversity values that deserve more rigorous protection.
Submerged aquatic plants – indicators of lake trophic status?
Submerged aquatic plants are sensitive indicators of environmental change as their growth and health is determined by the physical and chemical characteristics of the waterbodies in which they grow.
NIWA is investigating use of submerged aquatic plants in New Zealand as indicators of lake trophic status (the extent to which lake water is nutrient-enriched).
Find out more about submerged aquatic plants
To investigate the relationship between lake trophic status and submerged plant species we have analysed over 11,000 plant records contained within NIWA’s Freshwater Biodata Information System and data on the trophic status of NZ lakes. Overall, both submerged aquatic plant and trophic status records are currently available for 132 lakes. These lakes cover the range of trophic conditions from oligotrophic (nutrient-poor) to hypertrophic (very nutrient-rich).
Read more about our Freshwater Biodata Information System
Number of lakes in each trophic category
| Trophic status | No. of lakes |
|---|---|
| Oligotrophic | 34 |
| Oligotrophic-Mesotrophic | 7 |
| Mesotrophic | 42 |
| Mesotrophic-Eutrophic | 5 |
| Eutrophic | 29 |
| Supertrophic | 3 |
| Hypertrophic | 12 |
Species abundance versus lake trophic status
For each plant species occurring in a lake its abundance was calculated as a function of the number of sites it occurred at, the depth range that it covered, and the median cover it achieved. As a first step, we looked at the relationship between plant abundance (data for all plant species included) and lake trophic status. As expected, this analysis showed that the overall abundance of submerged vegetation in lakes tends to decrease as lake trophic status increases. This is explained by lakes becoming more nutrient-enriched and growth of algae or high levels of suspended sediments tending to ‘shade-out’ submerged plants, blocking light for photosynthesis and restricting their growth. In hypertrophic lakes particularly, water may be too turbid for submerged plants to grow.
For each individual submerged plant species we plotted species presence and abundance against lake trophic status to see if the plant species exhibited a preference for nutrient-poor or nutrient-rich waters. Based on the strength of the relationships seen we were able to determine the trophic preference and range for each plant species (see table of trophic preferences and range).
Table of trophic preferences and range
Our analysis also shows that alien aquatic weed species do not necessarily prefer more nutrient-enriched lake waters as is often thought. While Potamogeton crispus prefers more nutrient-enriched waters, Ceratophyllum demersum and Egeria densa also occur in some oligotrophic waters, and Lagarosiphon major and Elodea canadensis prefer less nutrient enriched waters.
Future work
In the UK a method has been successfully developed that uses macrophytes (aquatic higher plants) to assess river trophic status. This method is called the Mean Trophic Rank: Assessment of the Trophic Status of Rivers using Macrophytes (Holmes et al., 1999).
As in the UK, there is potential for a trophic ranking system to be developed in NZ based on the presence and abundance of aquatic plant species. While in the UK, the method has only been applied to rivers, our results indicate that a system could be developed for use in NZ lakes, and potentially for use in rivers as well.
NIWA has already developed a successful method for assessing overall lake ecological condition called LakeSPI (Lake Submerged Plant Indicators). However, it would be useful if the method could be adapted to simultaneously provide information on lake trophic status. In the future we aim to work with end-users to develop a simple ranking system that can be incorporated into our LakeSPI method.
References
Holmes, N.T.H.; Newman, J.R.; Chadd, S.; Rouen, K.J.; Saint, L.; Dawson, F.H. (1999). Mean trophic rank: A user’s manual. Environment Agency Technical Report E38. Environment Agency, UK.
Livingston, M.E.; Biggs, B.J.; Gifford, J.S. (1986). Inventory of New Zealand lakes. Part 1 North Island & Part 2 South Island. Water & Soil Miscellaneous Publications 80 & 81. Ministry of Works and Development, Wellington.
We would like to thank Michael Reid (NIWA Christchurch) for providing an electronic version of data contained within the Inventory of New Zealand Lakes (Livingston et al. 1986).
Macrophyte data over the internet
A NIWA initiative now sees freshwater biodata, including macrophyte records, available over the internet. This Freshwater Biodata Information System (FBIS) is designed as a safe repository for raw data, with the advantage of powerful data-discovery functionality.
Biodata information provides knowledge for decision making. How should water body managers and other interested parties use this facility? Here we introduce some of the capability and simple steps to extract macrophyte information.
Lake macrophyte data
The macrophyte data has been gathered from lakes by NIWA researchers using a SCUBA survey technique (see 'Submerged plant data: answering the what, where and why' for more detail).
Read our Water & Atmosphere article 'Submerged plant data: answering the what, where and why'
Currently vegetation records exist for up to 224 lakes, many surveyed more than once, totalling almost 20,000 plant records. Soon, any agency will be able to contribute a verified macrophyte record from any location as long as it is accompanied by a spatial reference and date.
Freshwater Pest plant data
Freshwater Pest plants comprise another macrophyte data set available in FBIS. This comprises over 2000 spatially referenced records for 44 of the worst freshwater pest plants in New Zealand. This initiative will provide a national overview of pest plant distribution in freshwater and trends in their spread.
FBIS functionality
Macrophyte data can be extracted using text type searches ('Common', 'Basic' or 'Advanced search') that allow the user to define particular attributes to search for and to choose attributes to display in any result. Alternatively, one may search spatially to discover the records in any area of interest by using 'map view', or by a combination of text and spatial searches.
The numbers of text records displayed are limited to the first 100, but you can request a count of all records and decide whether to download all records to an excel spreadsheet as 'comma separated values' (CSV).
Simple macrophyte searches
To get started, we suggest using the three common searches outlined below:
1. Discover if lake macrophyte data exists in your area of interest
Water managers may be interested in the records that relate to a particular region of responsibility. To find what lake macrophyte records exist in your region follow these steps.
Click on the 'map view' option listed across the top of the FBIS web-pages. Then, under the 'Search Tools' options (to the right of map), click on 'Common spatial search' and draw a bounding box around the area of interest (see example for the Rotorua Lakes – Bay of Plenty).
This will automatically lead to the 'Common Search' option. Choose the search 'Places where NIWA has surveyed aquatic plants'. The 'Search Results' are automatically displayed in a separate window, providing a full list of the lakes with macrophyte records that exist in the area of interest.
2. Extract macrophyte records for a lake
Managers require basic resource information on waterbodies for Environmental Reporting purposes. The most basic information on macrophytes in FBIS is a species list for any surveyed water body.
Click on the 'Common searches' option listed across the top of the FBIS web-pages'. Choose 'Species present at a given location'. For 'Area of science', choose Macrophytes from the drop-down box and click the << button. Type the name of the water body of interest in the box for 'Locality' (for example, Lake Pupuke). To ensure correct entry of the lake name, there is a 'Lookup Data' option on the right of the text entry box, where most lake names appear alphabetically under L. In the Data Lookup box, click the 'Select Locality' box to the left of the lake name and scroll down to the bottom of the page to click on 'Choose Data'. Finally, click on the Search button at the bottom of the page and search results will come up in a separate window.
Note that with so many waterbodies having the same name, it may be important to check you have selected the correct one by choosing the option of 'plot results on a map'.
If more detailed information is needed (e.g. plant depth ranges and covers on a survey date), FBIS provides the option of a Lake Vegetation Summary Report under 'Reports' on the menu bar. Note that you will be required to 'log in' to see these summary reports and a username and password will be made available on contacting the FBIS team.
Species distributions
It is often useful to view the distribution of a plant species. For example, the risk of a pest plant invading a waterbody may be judged by proximity to the nearest infestation. Alternately, the presence of geographically restricted (and potentially vulnerable) native species can be viewed.
To obtain species distributions, choose the option 'Common searches' listed across the top of the FBIS web-pages. Choose 'Species Distribution'. Type your choice for 'Genus' (and 'Species') in the 'Search Criteria' box, (weedy curly pondweed in this example). Click on the Search button at the bottom of the page and search results will come up in a separate window. Finally, in the Search Results window, choose the option of 'Plot results on a map' to display the macrophyte distribution on a map.
Use of FBIS information can provide:
- A description of vegetation resources (e.g. native character, rare plant presence, potential pest plant problems)
- An indication of change in lake condition
- An assessment of local weed risk based on proximity of national pest plant distribution
We would appreciate that any presentations or documents using FBIS data include an acknowledgement of FBIS, preferably including the web address, fbis.niwa.co.nz. Data provided from FBIS are not to be used for commercial purposes without NIWA’s written permission.
This project was funded by Department of Conservation’s Terrestrial & Freshwater Biodiversity Information System (TFBIS) Programme), and the New Zealand Foundation for Research, Science and Technology (CO1X0221), with lake surveys co-funded by a range of agencies.
Contact
