To undertake surveys, surveillance and monitoring of Zostera biotope attributes, site managers will need to identify the most appropriate and cost-effective field and analytical methods, as well as determining the quality assurance requirements. Analytical methods include Geographical Information Systems (GIS) and Remote Sensing Information Systems. A major advantage in using a mixed monitoring strategy, employing a combination of the methods outlined below, is the production of more accurate maps allied with the increased flexibility of interpretation and query within GIS.

Techniques that can be used to monitor Zostera biotope attributes are listed below. In the following sections, each technique is briefly described, and its advantages and disadvantages summarized. Examples of the use of particular methods are given.

Aerial remote sensing techniques include aerial and infrared photography, satellite sensor images and multi-spectral scanning imagery (CASI).

A vertically mounted camera on a light aircraft takes high resolution, large format, digital natural colour transparencies, in transects across the site. Using infra-red, the methodology is the same, but this format allows better differentiation between intertidal algae and Zostera. The advantages and disadvantages are summarized below:

BKS Surveys Ltd. tested the usefulness of aerial photography for mapping Zostera beds in the Isles of Scilly SAC and at Lindisfarne and Budle Bay (within the Berwickshire/ North Northumberland SAC) in 1996. Infra-red photography was found to be more effective than natural colour, but difficulties were experienced in distinguishing between living and dead material, and in distinguishing Zostera from the alga Enteromorpha (P. Gilliland, pers. comm.). The Isles of Scilly aerial survey was ground-truthed by the Coral Cay Conservation Sub-Aqua Club in 1997. The technique was found to be valid but the density classes were found to be optimistically high (Irving et al., 1998).

Images from satellite sensors (Landsat Thematic Mapper & SPOT XS) can be used for a number of mapping applications. However, the habitat classification accuracy is highly dependent upon the methods used and different habitats may not be accurately distinguished (Mumby et al., 1997).

The Compact Airborne Spectrographic Imager is a digital airborne sensor providing high spectral and spatial resolution. It has been used for a number of mapping applications, principally on tropical reefs and seagrass beds. This sensor is mounted on a light aircraft and can be flown for example, at 3000 m giving 4 million pixels in 15 bands, and at 750 m for 1 million pixels in 8 bands. This provides considerable mapping accuracy and its application for mapping Zostera biotopes is under review. Initial trials suggest that predictions of standing crop using CASI give results to a similar order of magnitude to quadrat sampling in situ (Mumby et al., 1997). CASI is now being tested and used by the Environment Agency, SEPA and water companies.

Acoustic Ground Discrimination Systems (AGDS) are a comparatively recent development in through-water remote sensing and are becoming increasingly important in the large-scale mapping of benthic habitats and communities. The two principal techniques relevant to the mapping of subtidal Zostera biotopes are RoxAnn^TM and side-scan sonar.

RoxAnn^TM is an electronic system using a sonar signal. The first and second echoes returned from the seabed are re-analyzed. This analysis derives values for the roughness and hardness of the seabed. By integrating these data with other information on water depth and position, a map of the physical characteristics and distribution of substratum types can be produced. The biotic characteristics of many marine communities will predictably affect the values recorded and consequently, it is possible to map the distribution and the extent of these characteristic benthic communities. An essential part of any AGDS survey is to adequately ground-truth the data to confirm the habitats and communities mapped. Differential GPS can be used for position fixing.

When using RoxAnn^TM around the Isles of Scilly, it was possible to clearly differentiate between dense Zostera beds and sand, but the beds could not be distinguished from alga-covered rock or gravel areas. Side-scan sonar clearly demarcated dense Zostera beds with eroding margins but was insensitive to sparse Zostera beds (Munro & Nunny, 1998).

The use of video for underwater survey is becoming increasingly important as it allows a permanent record of many aspects of benthic biotopes to be kept. Three remote video surveying techniques can be employed in the study of Zostera beds, Remotely-operated vehicles (ROVs), towed video and drop-down video.

The ROV is the most versatile system, as it is a mobile vehicle that has complete three-dimensional movement in the water and is highly manoeuvrable. A high quality camera and lighting system allows good quality video to be obtained. An ROV can be operated at a height above a Zostera bed, and flown along a transect to obtain data on the distribution, extent and boundary dynamics of the bed. It can also be flown into the bed to obtain data on plant condition, bed density and associated species diversity.

A towed video camera is mounted on a light-weight, metal sledge that is towed at a known speed over the seabed by a boat. This method can provide information on the extent of a Zostera bed, and may be able to gather additional information on the bed’s boundary dynamics. However, one disadvantage of the technique is that repeated passes of a towed sledge through a Zostera bed may cause some physical damage.

The drop-down video is the simplest and cheapest of all three remote video surveying techniques. It consists of a video camera in a waterproof casing, mounted in a simple metal frame. The camera is held off the seabed and points down and forwards. When deployed, the camera will obtain spot information over a small field of view, allowing identification of the Zostera biotope in that location, and providing an indication of the plant density and associated community. The system is quick to deploy and recover.

Grabs and cores can be used to sample Zostera beds. However, as a destructive sampling technique, with the potential to cause damage to the beds, their widespread application as a monitoring tool is likely to be restricted to targeted studies, relating to plant status and aspects of infaunal community structure.

A variety of grabs and cores can be employed on the shore and from boats. Those commonly employed by biologists in the intertidal and shallow sublittoral zones are tube-corers, Van Veen grabs and Day grabs. The data obtained from these samples can provide useful information on the substratum type, plant condition, plant and rhizome density, sexual status and infaunal community composition. It is possible to determine the location of a sublittoral Z. marina bed and to establish its approximate extent using a series of grab transects. However, this is likely to be both time-consuming and destructive.

An experienced and skilled field biologist, with sufficient time and resources, will often provide the best quality data when monitoring complex communities such as Zostera biotopes. The remote sensing and sampling techniques outlined above will provide quick and cost-effective data over a large area, for many Zostera attributes, particularly distribution and extent. However, many aspects of detailed ecological monitoring of Zostera biotopes require hands-on fieldwork, both intertidally and subtidally.

An indication of the quality of a subtidal Z. marina biotope and its associated community can be provided by the remote video techniques outlined above, most successfully using an ROV. However, to obtain more comprehensive information on the species diversity, including the presence of characteristic and representative species, surveying by experienced diving biologists will be required.

There are many techniques that divers can employ, including MNCR Phase II and III surveys, which involve taking core and grab samples for later analysis. In addition, targeted studies and monitoring of key attributes can be undertaken.

Intertidal field biologists can collect monitoring data for the majority of intertidal Zostera attributes, often in the same site visit. MNCR Phase II and III survey methodologies can be employed. Samples can be collected, and remote sensing can be accurately ground-truthed.

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