Water Plant Surveys at Wheatfen
Dr Dan Hoare Conservation Officer (Waterways), Broads Authority
Water plants are an often-overlooked part of our flora, as their submerged life
and relative inaccessibility makes them sometimes rather tricky to see. This is
not helped by cloudy water and dense marginal growth frequently obscuring them
from the gaze of the casual observer. Nonetheless, the Broads Authority, through
its annual water plant survey, routinely monitors submerged plants present in
the open water of many of the broads. The underwater plant growth provides a
valuable habitat for a range of aquatic organisms, especially invertebrates, which
can feed upon algae growing on the plant surfaces, shelter from predators within
the tangled physical structure, or utilise the stems and leaves to lay their eggs
upon. Water plants also help to improve water quality, through stabilising bottom
sediments and using up dissolved nutrients, which algae would otherwise use
and turn the water green.
The open water and channels of Wheatfen are a prime example of relatively
undisturbed aquatic habitat in Broadland where water plants can thrive. Sunlight
can penetrate the water to the bottom sediments, as the water depth is never
too deep, thus enabling new seedlings to sprout and grow. Fortunately, water
quality and clarity in the River Yare has been progressively improving over the
last decade, so the potential for submerged plant growth has increased in recent
years. Specific designation of the open water habitats of Wheatfen, as part of
the Yare Broads & Marshes Site of Special Scientific Interest (SSSI), means that
water plant monitoring is a crucial part in assessing the long-term changes in the
ecological health of the area.
The Broads Authority water plant survey has been conducted four times at
Wheatfen in the past nine years. The sampling method for collecting submerged
plants entails dragging a double-headed rake attached to a long rope, along
previously mapped transects. Retrieval of the rake at the end of each haul
often produces a surprise or two. This year a yellow water lily seedling was
particularly nice to see, especially as it was still attached to its seed case.
Clearly these plants are reproducing sexually, as well as regenerating from their
over-wintering rhizomes. The odd swan mussel is temporarily disturbed, as was
one of the relatively new arrivals to the Broads, an Asiatic clam.
Several different growth forms of aquatic plant are now found commonly at
Wheatfen, where perhaps a less diverse mix was found a decade or more ago
when the water was more murky. As previously mentioned the water lilies are the
most obvious growth form, with their large floating leaves and colourful displays.
Less showy, but with a certain refined grace, are the dense clumps of starwort
which are particularly common in the channels and ditches. Their thin pale-green
leaves form a delicate four-pointed star shape when viewed in cross-section. The
tidal ebb and flow through the restricted channels also creates suitable conditions
for the submerged strap-like leaves of the unbranched bur-reed, a species often
seen in larger river channels, with its leaves coming up vertically from the bottom
sediments. As many of the channels at Wheatfen have been dredged in recent
years, increased amounts of plant varieties forming dense underwater beds are
evident. Species such as rigid hornwort and Nuttall’s waterweed, with compact
leaf structures and tousled stems, have colonised the once bare mud and
created bushy green pillows within the clear water. The variety of growth forms
is as important as the number of species found, and is certainly an improvement
on the blanket-weed and stringy algal mats that were found to coat much of the
area as recently as 1998.
The gradual change in aquatic plants, both in species type and overall vigour,
which has been observed at Wheatfen, helps to justify the often large expenditure
that accompanies restoration and conservation efforts in these fragile habitats.
The benefits are also not purely those that may interest the botanist, but
permeate through all parts of the aquatic ecosystem, as exemplified by numerous
sightings of a top Broadland aquatic predator at Wheatfen, the otter. Having
trawled nearly 70 kilometres with the rake last summer, including sampling in
the rather barren River Bure broads whilst dodging holiday boats, coming to
Wheatfen was a real treat. Who knows what we will find next year?
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