ENSIS was contracted to undertake this assessment on sediment cores taken from the Broads. Additionally, sediment analysis can be used to determine the extent and composition of the past macrophyte communities that populated the sites prior to eutrophication and thus provide accurate baselines for use as restoration targets. ENSIS will provide this information to inform the de-silting procedure by providing the optimal depth(s) of sediment to remove in order to maximise both nutrient removal and propagule viability.
Objectives:The objective of the project is to to determine pre-eutrophication aquatic plant communities in Hoveton Great Broad and Hudson’s Bay and to determine seed and oospore viability in the sediments of the two broads
The main aims of the project were to:
Sediment core collection. Sediment cores were taken at 32 separate locations from the two broads. A single core from each location was extruded to provide large volumes of sediment for the germination experiments. At seven locations a second core was taken to provide sediment for analysis of organic and carbonate content and elemental geochemistry. These master cores were also sub-sampled for macrofossil analysis. Organic and carbonate content were determined to understand the variation in sediment composition across the sites. Total phosphorus within the cores were also measured using X-ray fluorescence (XRF).
Macrofossil analysis Surveys of the current aquatic macrophytes in the two broads were carried out in July 2014 using JNCC Common Standard methodology. The surveys consisted of four components; a strandline survey, emergent and marginal survey, shoreline “wader” survey and boat survey. To enable comparison with historic survey data, estimates of species abundance for each set of survey points at a site were made from the total number of occurrences, and these data have been converted to a DAFOR scale.
Contemporary macrophyte survey Surveys of the current aquatic macrophytes in the two broads were carried out in July 2014 using JNCC Common Standard methodology. The surveys consisted of four components; a strandline survey, emergent and marginal survey, shoreline “wader” survey and boat survey. To enable comparison with historic survey data, estimates of species abundance for each set of survey points at a site were made from the total number of occurrences, and these data have been converted to a DAFOR scale.
Germination Experiments Germinations experiments were conducted in 12 litre aquaria over a 6 month period. The 10 cm sections of sediment core taken from 30 cm to 80 cm depth were individually homogenised and spread in the base of a clean, pre-labelled plastic aquarium. In addition, the upper three 10 cm sections (0-30 cm) and basal section (c. 90-100 cm) from each of the 7 master core locations were also placed into aquaria. Each aquarium was filled with 10 litres of river water, thus providing conditions similar to that of the broad. The 184 aquaria were then left for six months and observations made of the numbers and species (or type) of germinations.
Sediment geochemistry. Results are presented in the final report where detailed core stratigraphies are available covering sediment P, organic and carbonate concentrations together with a range of other mineral elements (Pb, Al, Si and Ti) measured by XRF. For HGB, there is an increase in P concentrations in all the cores with the values at least doubling between the base sample and upper 5 cm of the cores. The general trend within all the cores is for the P values to follow the same pattern as changes in % LOI, indicating that P concentrations are in the most part associated with organic matter. The two Hudson Bay cores also exhibit an overall increase in P concentrations in the two cores with similar changes in concentration and association with changes in % LOI, suggesting that P concentrations are in the most part associated with deposition of organic matter and reflect the lake productivity.
Plant macrofossil analysis. None of the five cores analysed from Hoveton Great Broad for plant macrofossil remains were particularly species rich in terms of remain types, nor were there many plant seeds or propagules recoded. The cores fall into two groups. Firstly these dominated by the leaf and spine remains of Ceratophyllum demersum through most of the levels, with M. spicatum leaf tips and seeds also present more commonly in the lower samples. The presence of water lily trichosclereids coincides with C. demersum remains in these cores and although other aquatic species are recorded, numbers are very low. The second group have Chara spp. oospores appearing more commonly in the lowest samples, with C. demersum more abundant towards the upper samples. These data suggest Hoveton Great Broad to have had a mixed aquatic flora in the past and one typical of eutrophic waters. Interpretation of the exact nature of the shifts in community composition are made difficult by low sample numbers and difficulties correlating the cores, but as seen in previous work there appears to be a shift from Chara dominance to mixed M. spicatum, C.demersum, fine leaved Potamogetons and Chara to dominance to by large areas of mixed lily beds.
The two Hudson Bay cores are quite similar in their plant macrofossil records. Neither core had any Chara oospores recorded, suggesting that sedimentation rates were significantly higher in this more sheltered basin and thus the 100 cm cores do not extend back to a time when Chara was present in any quantity. Below 50 cm there is good evidence to suggest the broad supported a mixed aquatic flora with C.demersum likely to have been common, alongside fine-leaved Potamogeton spp as well as water soldier (S. aloides), Callitriche spp. and water lilies. Above 50 cm, there appears to be a marked change from submerged species to floating leaved taxa. Water lily trichosclereids become more numerous, C. demersum almost disappears and the presence of Azolla appears common within the record.
Contemporary Macrophyte Surveys. With the exception of some relatively large beds of Nuphar lutea and few smaller beds of Nymphaea alba, HGB has only very sparse aquatic vegetation. Ceratophyllum demersum was the most commonly recorded species, but it was absent from much of the open water, but found growing over a relatively wide area to the north-east of the broad. Rarely did more than a single plant appear on the grapnel, although where present most 4 m grapnel hauls would yield a plant. Potamogeton pectinatus was recorded only very rarely in the east and north of the lake. There remain large areas of the open water in the central and south-west of the site that had no submerged plants. Lemna minor was seen growing in sheltered areas of the bank, mainly within stands of rooted emergent plant e.g. Carex acutiformis, C. riparia and Phragmites australis.
Much of the south end of Hudson’s Bay is less than 50 cm in depth and without any aquatic plants. The sediments is very fluid and easily resuspended thus making them it rooting media for most species. The broad opens out more towards the northern end and reached depth of 90 cm. White and yellow water lilies are common in the northern half of the site with N. alba being slightly more frequent. Other species were rare in the site.
Germination results. From the 184 aquaria a total of 234 germinations have been recorded. Of these, only seven were angiosperm plants. The other 227 germinations were all charophytes.
Hudson’s Bay is rather different to Hoveton Great Broad with fewer Chara spp. germinations and those that did germinate, did so from lower in the cores. The low numbers of Chara germination from the Hudson’s Bay cores is perhaps not surprising given the lack of any oospores recorded from the macrofossil analysis. It is also clear from the germination data, that not only do germinations vary with depth, but also by locality in the site. Within HGB, there were very few germinations from the eastern end of the broad at any depth in the sediment, whereas the central part of the broad had relatively high numbers of germinations from 30 to 50 cm. Again, these results are borne out by comparison with the macrofossil data that show there to be very few oospores in the eastern side of the broad.
Evidence of past plant communities and habitats This and other studies show that the site had greater aquatic plant diversity in the past than the site has supported in the last c. 50 years. Lower portions of the sediment cores had remains of stoneworts (Chara spp.), fine-leaved pondweeds (e.g. Potamogeton pusillus type), horned pondweed (Z. palustris), rigid hornwort (C. demersum), Holly-leaved naiad (Najas marina) and Water milfoil (M. spicatum) and both white and yellow water lilies (N. alba and N. lutea).
The reduction in macrophyte species composition and abundance can almost certainly be linked to changes in water quality and clarity. Previous palaeoecologiocal analysis of algal communities shows a significant shift from benthic communities, typical of clear water conditions, in basal sediments to assemblages dominated by planktonic species towards the top of the cores. This palaeoecological and historic evidence demonstrates the link between good water clarity and increased plant diversity and therefore the need for clear-water conditions to be re-established at these broads.
Sediment composition and stability Clear water conditions are not simply a function of planktonic algal biomass however, but are also dependent on suspended solids, which in a large, shallow site such as HGB are likely to be exacerbated by the re-suspension of material from the lake bed. The likelihood of this in hyper-eutrophic lakes is increased due to high sedimentation rates, flocculent organic sediments and lack of consolidation by plant roots. HGB has not always consisted of open water as it does today, but in the past, had large areas of floating “hover” breaking the site into a mosaic of more sheltered ponds. It is suggested that the site existed like this in the early part of the 20th century. There is evidence to suggest that the Bure Broads were already experiencing rapid eutrophication by the 1900s, and the loss of plant species in HGB may have coincided not only with increased planktonic algal production, but also the opening up of the site due to loss of the hover and shelter it provided.
Sediment stability and structure may therefore be important in future restoration work with the removal of the uppermost organic sediments helping to reduce sediment re-suspension. The organic content of all the cores analysed from HGB and Hudson’s Bay showed the uppermost sediments to be the most organic, with a slight reduction between 15-50 cm, followed by a second peak lower down. If de-silting is to take place, it is recommended therefore that some consideration is given to determining the depth of sediment to be removed that will expose more consolidated material that is less likely to be re-suspended and also provide a more stable substrate into which aquatic plants can root.
The HGB and Hudson’s Bay cores show an increase in sediment phosphorus concentrations in the uppermost samples. These data broadly corresponds with figures for sediment P for the Bure Broads. It is clear however, that although sediment P is at its highest at, or just below the surface sediments for the majority of cores, concentrations rise to similar levels at depths ranging from 30-50 cm; the target depth suggested for sediment removal.
Sediment removal would remove P from the system, but there is no reason to expect a reduction in the exchange of P from the sediment to water following dredging. Evidence from previous de-silting studies and laboratory-based experiments have shown only short term reductions of internal P loadings for partial sediment removal, after which the sediment P equilibrates with the overlying water and flux rates approach pre-intervention levels.
Given the relatively high concentrations of sediment P within the target depth of de-silting, there is no evidence to suggest dredging would reduce internal P loading. Irrespective of de-silting at these broads, efforts should remain focused on continued reductions of external P loading to the sites. Isolation from the River Bure may help to prevent relatively nutrient rich waters entering HGB and Hudson’s Bay, but this also greatly reduce flushing rates within these broads and potentially result in increased P in the water. Hydrological budgets and nutrient budgets from the feeder streams / ditches to the north of the broads are therefore recommended to assess post-isolation nutrient budgets for the sites. The role of the “seed bank”
This is the first study that we are aware of that attempts to ascertain the distribution and viability of a “seed bank” prior to undertaking sediment removal in a lake. In its simplest form, what the results show is where and to what depths viable plant and macro-algal propagules exist. They also demonstrate the erratic nature of the dispersal of Chara oospores within a site and the disparity between presence of oospores and viability.
The germination experiments show there to be relatively high viability of Chara oospores, even at sediment depths of over 50 cm – probably equating to 50-100 year old (and older). Whilst the Chara oospore viability is of ecological interest, it is not particularly important to the restoration of HGB and Hudson’s Bay because this is not considered to be the restoration target for the sites. Furthermore, despite being viable within the seed bank, charophytes require relatively high water quality to persist within lakes, and these parameters are not currently met, particularly for nitrogen, which remains high in the River Bure system.
What the study does show, is that the removal of up to 50 cm of sediment from these broads is unlikely to threaten any viable seed bank. Factors such as good connectivity to the surrounding wetland areas and ditch system to the north of the sites are likely to provide the best mechanism for the arrival of seeds and propagules to the broads.Irrespective of how propagules arrive at the site, it will be crucial to maintain clear water conditions within the site for several years in order for aquatic macrophytes to become well established. There is therefore a trade-off between water quality (and clarity) and the depth of silt removal. Light attenuation is rapid where waters are turbid and thus removing too much silt may deepen the sites beyond the depth at which plants can establish. Removing too little silt and plants are more susceptible to grazing by non-diving ducks and swans.