Macrophytes are the larger plants of fresh water, readily distinguished by the naked eye and include angiosperms (flowering plants), pteridophytes (ferns, horsetails and quillworts), bryophytes (mosses and liverworts), charophytes (stoneworts) and filamentous algae.
The macrophyte flora of each site is sampled in an easily replicated fashion so that broad qualitative changes in both floristic composition and relative abundance can be assessed. Within the period mid-June to September each stream is sampled annually while lakes are sampled biannually, and, since funding cuts, triannually. Sampling is undertaken centrally by ENSIS-ECRC at UCL and quality control is ensured by the collection and preservation of voucher specimens and the exchange of specimens with botanical experts based at the Natural History Museum London, The Royal Botanical Gardens Edinburgh and the University of Bergen.
Each lake is sampled by three methods:
As much of the inshore zone as possible is viewed either by walking the shoreline, wading or from a slow-moving boat. Emergent, floating and submerged macrophytes in the shallow inshore zone are recorded and major stands annotated on to a large scale map. Major vegetation types fringing the lake are also recorded.
Two transverse trawls (four in larger lakes) are made across the lake by trawling a grapnel (double headed rake) attached to a long rope behind a boat travelling at a steady speed. Each traverse is sub-divided into five approximately equal trawl sections for which the amount of plant material recovered and relative abundance of individual macrophyte taxa are estimated. [Trawls are carefully situated away from coring sites and sediment traps.]
Three sites in each lake (four in larger lakes) are chosen for more detailed survey transects of 50-60 m in length aligned perpendicular to the shore. At least one transect is located on a steeply shelving and/or exposed shore. A fixed line is deployed along the transect and Ekman grab samples are taken from the sediment surface at 10 m intervals with an additional site 5 m from the shore. Water depth, substrate type, amount of plant material and relative abundance of species are recorded for each Ekman grab sample and the exercise is duplicated at each sample point along the transect.
The location of end stations of both lake trawls and transects are recorded by GPS to ensure that subsequent sampling occurs in the same areas. Since the trawls and transect survey are destructive sampling methods, sampling exactly along previous survey lines is avoided as far as possible. In practice however the process of setting the transect can result in overlaps between visits.
The combination of the three survey methods provides information to assess the relative abundance of individual macrophytes occurring in each lake. Moreover, the profiles generated by the transects indicate the approximate maximum depth to which living macrophytes extend. This information is transferred to the central biological database and summarised in the annual data reports of the Network as a list of taxa together with DAFOR abundance estimates.
Since 2009 at lake sites the Common Standards Monitoring Methodology for lake macrophyte sampling has been performed alongside the UK UWMN protocol.
Dry-weather flow is a pre-requisite for sampling in-stream macrophytes. A 50 m section of stream containing a representative range of aquatic macrophytes is selected. Every 5 m from 0-50 m inclusive, a transect is laid across the water-filled section of the channel and water depth, substrate and macrophyte taxa (if any) are recorded at three equidistant points along that transect. In the 5 m stream sections between each transect the stream bed is surveyed and the total amount of plant cover (expressed as a percentage of submerged stream bed) and floristic composition of the plant assemblages are estimated visually; the substrate composition of the stream bed is also recorded in these sections, which are easily replicated in subsequent surveys.
Major morphological features and the location of notable growths of plants in the channel are annotated on to large scale sketch maps. Plants growing in the channel and on the banks not submerged during dry-weather flow are also recorded. All submerged plants are recorded; data are transferred to the central biological database and appear in the annual data reports of the Network as a summary list which indicates the estimated percentage of submerged stream bed throughout the 50 m length covered by each taxon.