Najas marina

Najas marina L. (Najadaceae)

Holly-leaved naiad, Hollyweed

Status in Britain: VULNERABLE. WCA Schedule 8.

Status in Europe: Vulnerable.

This aquatic species occurs in a few Norfolk Broads where it is confined to clear, mesotrophic and relatively unpolluted water. It is usually found in sheltered waters with minimal boat traffic since turbulence or strong wave action renders its brittle stems vulnerable to breaking. The late development of the 'leaf canopy' means that it cannot grow well alongside more aggressive waterweeds which mature much earlier, as is often the case in more nutrient-rich habitats. N. marina tends to grow best at water depths of between 0.5 and 1.5 metres, generally rooting into fine organic sediments. It may form pure stands, or grow with such species as Ceratophyllum demersum, Myriophyllum verticillatum, Potamogeton pectinatus, or charophytes including the rarities Chara intermedia and Nitellopsis obtusa. In some places it can form mono-dominant stands in which filamentous algae may be its only associates. Areas of bare substrate are readily colonised where conditions are suitable.

N. marina is a dioecious annual or short-lived perennial, a most striking and distinctive plant when mature. Male plants have, however, not been detected in Britain. It is fully developed in mid- to late summer, and flowers in July and August. Fruit is common, and it sets good seed in the absence of male plants, suggesting the female plants may be apomictic.

This plant was formerly much more widespread in Broadland and has occurred in most of the Broads. Outlying sites in the last 40 years include Alderfen Broad, Barton Broad and Hoveton Great Broad, but it occurs in none of these today (Jackson 1981). It is now largely restricted to the Thurne valley, and is particularly abundant in Hickling and Martham Broads. In Heigham Sound it is very scarce and has been found only a few times recently despite much searching. Blackfleet Broad supported good populations in 1985 but encroaching reedswamp and increased sedimentation are making the site less suitable. It is likely to be surviving in a number of the ponds scattered amongst the extensive marginal reedswamp of the Upper Thurne. In Upton Broad, an isolated site in the Bure valley, N. marina is locally dominant, and it has now reappeared further upstream at Pound End and Cockshoot Broad, where it is increasing (Kennison 1993).

In recent years, nutrient loading has been reduced by phosphate stripping at key sewage treatment works, and some Broads have had their nutrient-rich sediments removed by suction-dredging. Experimental lake management through biomanipulation of fish populations has recently brought about the clear water that N. marina requires, and the strengthening colony at Cockshoot Broad bears testimony to the success of this work. N. marina has seen a small recovery in the 1990s, but as poor water quality can lead to a very rapid decline, this plant remains vulnerable. 

N. marina is a cosmopolitan species, occurring in both temperate and tropical regions in Europe, Asia, Africa, Australia and North and Central America. It is a variable plant, and many sub-taxa have been recognised in the Old World (Triest 1988). The British plant is ssp. intermedia. Though N. marina (also including ssp. marina and ssp. armata) is widespread in lakes across Europe, it is rare or endangered in many regions, particularly in northern Europe.

G. C. B. Kennison