Alkborough Managed Realignment

Figure 8 shows that between September 2006 and December 2007 stations with greater numbers of inundations experienced greater rates of accretion (up to 0.6m).

The areas of highest elevation and accretion are generally situated around the distribution channel where elevation is predominantly greater than 3.2 m, reaching a maximum of 3.4 m in some part of sectors 1, 4 and 5 (Figure 7). In 2008, elevation in most of this area was 3- 3.2 m with small areas being 3.3 m. A significant increase in elevation has occurred in most sectors (except 3 and 5a) since 2007 although the rate of accretion appears to be decreasing over time. Differences between 2008 and 2009 were not significant. Elevation outside the realignment site is variable over time with the highest elevation being recorded in September 2008. Elevation for March 2008 and September 2009 was equal. It is of note that this area undergoes frequent cycles of erosion and deposition, as indicated by the steep and terraced nature of the mudflat.

Like other realignment sites on the Humber, the initial rate of accretion at Alkborough has been rapid but is now stabilising to some extent. Average accretion ranged from 0.02 m to 0.09 m, compared to values of up to 0.06 m during the first year. Maximum accretion is predominantly around the distribution channel and it is suspected that a high level of accretion has also occurred within the channel. Accretion levels outside the site are stable (statistically) and most of the area has remained at an elevation of 0 – 2.6 mOCD. It is apparent that the mudflats in this area undergo frequent cycles of erosion and deposition.

New analysis undertaken by Halcrow (in prep 2012), of the yearly elevation data that is collected for the site, confirms the previously reported tendency for the majority of the site to act as a sediment sink and accrete. The initial rapid siltation from 2006 to 2008 (0.6m) has not continued and the average rates of accretion between 2008 and 2012 range between 0.03m and 0.09m per year. For most sectors siltation rates appear to have been relatively constant since the March 2008 survey.

Infauna

Infaunal species diversity was low in 2009. A total of 21 species was recorded across both sites, with a total of 19 species found inside the realignment site and 6 species of benthic invertebrates recorded outside the realignment site. Typically for upper estuarine areas, species diversity was low at all stations. Stations with the lowest numbers of estuarine species present were in the dry parts of the site. There were no clear spatial patterns in organism abundance when considering all species and mean abundance was variable across the site.

In 2009, biomass across the site was low with (<10 g m-2 at most of the stations). Biomass was low outside the realignment (<1 g m-2 at three stations). The low biomass ratio at all stations indicates dominance by small-bodied organisms. No significant differences were found between the abundance inside the realignment site as a whole and the established mudflats outside the site.

Inside the realignment site, species from the order Collembola were the most abundant and were recorded at 89% of stations. This order was also dominant in terms of biomass. The highest numbers of Collembola were recorded at station 401 with over 444,795 individuals m-2. The estuarine oligochaete Paranais litoralis was the next most abundant species recorded at 89% of stations. This species was the dominant species at 13 out of the 26 stations. Other taxa present at most stations within the realignment site included Enchytraeidae and other Oligochaetae worms, Nematodes, Ostracods, and insects from the taxa Ceratopgonidae, Chrionomidae, Diptera and other larvae unidentifiable to the species level. Outside the realignment site, the estuarine oligochaete Heterochaeta costata was the most abundant species and was recorded at all stations. It is of note that this species was not recorded inside the site.

Multi-dimensional scaling (MDS) showed a separation between the benthic communities inside and outside the realignment site. The separation between the two areas is still strong when only estuarine species are taken into account although some stations from outside the site (11 and 13) appeared to be connected with stations inside, but this was mainly due to presence of oligochaete Paranais litoralis. Generally, the differences between the two communities can be attributed to the presence of the freshwater/terrestrial species in the samples together with the comparatively low abundance and diversity of estuarine species inside the realignment site.

Species richness, abundance and biomass have increased notably at some of the stations inside the realignment site (201, 302, 303, 305 and 401) since 2007. Stations 201, 302 and 303 are situated close to the distribution channel, whereas station 305 is situated further from the channel. Only abundance and biomass increased at stations 601 and 603, which are situated close to the channel. Species richness, diversity, abundance and biomass at other stations inside and outside the site have fluctuated with no clear spatial or temporal patterns. Species richness inside the site as a whole increased (for all species and for just estuarine species) significantly between 2007 and 2009. Species richness outside the site has not changed significantly between these years.

Abundance inside the site increased significantly between 2007 and 2009 (for all species and for just estuarine species). Abundance inside the site (considering estuarine species only) did not change between 2008 and 2009, although increased significantly between 2007 and 2009. Abundance increased significantly outside the site between 2007 and 2009. Diversity of all species has not changed inside or outside the realignment site between these years. Considering only the estuarine species, diversity increased significantly between 2007 and 2009 and between 2008 and 2009. Biomass of all species significantly increased inside the site between 2007 and 2009, but no significant changes were detected between 2008 and 2009. Biomass of estuarine species significantly increased between these years.

Vegetation

A series of 2 m x 2 m quadrat surveys was carried out in areas where significant amounts of vegetation had colonised in order to determine species composition and to identify areas of initial colonisation (Figure 9). Altogether, quadrats were described at 15 transects. Two transects were run from the High Water Strand Line (T1 and T2) to the pioneer zone, others were linked to the benthic sampling points and situated inside the realignment site.

Whilst there were significant areas of terrestrial vegetation remaining (principally Lolium perenne), areas of salt tolerant vegetation are clearly developing. As sediment accretion proceeds, the spatial distribution and density of L. perenne is expected to decrease. The distribution and density of Puccinellia maritima (common saltmarsh grass), Aster tripolium, Juncus inflexus, Carex otrubae and Phragmites australis increased compared to previous years indicating that saltmarsh species are continuing to colonise the site. There were 12 saltmarsh species noted in 2009, an increase of 6 on the 2008 survey, and further indication of the transition from a terrestrial to a wetland site.

The most abundant and widespread plant species was Phragmites australis which was present all over the site. P, australis was already growing along the drainage ditches and this presence has probably aided the quick colonisation of other site areas through seeds blowing onto the mudflat.

Birds

А total of 13 species of wader was recorded between October 2009 and March 2010, which is three species less than the previous year. In comparison to 2008/2009, oystercatcher was found in 2009/2010, but knot, little stint and ringed plover were not recorded. The most frequently occurring species was redshank which was recorded feeding on the site on 42 of the 84 sampling occasions, although this species was only recorded using the site for roosting once. Lapwing were observed feeding on the site on 24 (out of 84) occasions and roosting on 17 occasions. Curlew were recorded feeding on the site on 24 occasions and roosting on 16 occasions. Dunlin were also recorded frequently at the site. Other species were recorded on less than 10 occasions, either feeding or roosting.

A total of 18 species of wildfowl was recorded, which is one species less than last year. Spoonbill and whooper swan were not recorded in 2009/2010, but one new species, the brent goose, appeared on the site. Shelduck was the most frequently occurring species, recorded feeding on 51 occasions and roosting on 28 occasions. Teal was seen feeding on 25 occasions and roosting on 35 occasions. Other frequently occurring species were wigeon which were recorded feeding on 23 and roosting on 22 occasions, and mallard which was recorded feeding on 19 occasions and roosting on 25. Other commonly recorded species were grey heron, teal and graylag goose.
The maximum total number roosting waders was 13164 birds recorded in December 2009 (including 6350 count for lapwing and 6800 count for golden plover). The results indicate that greater numbers of wildfowl used the realignment site for feeding, but greater numbers of waders used it for roosting.

Principal Coordinates Analysis (PCoA) was carried out to detect any spatial and temporal patterns in the distribution of birds across the site. It is of note that the some of the sectors contain contrasting habitat types and therefore attract birds with a wide variety of habitat requirements. However, when maximum and cumulative data are plotted, there is a strong association between bird species and habitat preference. The figures indicate that sector 6 (a grassy area) is used as a feeding area by curlew only, and another grassy area in sector 7 is used by curlew and lapwing. Other species of waders are primarily using areas 1, 2, 4 and 5 (frequently flooded areas of developing mudflat). The most important sectors for roosting waders appear to be sectors 1, 4 and 5, which are also the key wader feeding areas.

Fish

Water quality parameters reflected the normal temperature and salinity range expected in upper estuary zones and showed a degree of seasonal variability. Salinity can be extremely low inside the site, reaching as low as 0.3 and it is probably driven by river flow rates and rainfall. Small differences in temperature were noted between realignment (RL) and river bank sites (CTRL) probably reflecting the more variable conditions inside the realignment site resulting from the shallow and enclosed environment.

Fyke net sampling found that the species composition was reflective of the oligohaline nature of the area and was dominated by estuarine species adapted to low salinity (i.e. flounder and gobies), diadromous species (i.e. eel and smelt) and freshwater species. The remaining species included fresh water species or estuarine fish, most of which were only captured very occasionally. Among the more abundant freshwater species, 3-spined stickleback, chub and roach were relatively more prevalent inside the site. Breams (common and white) were also found in moderate numbers. Of interest is the presence of perch, a predatory fish, which indicates use of the site by prey species since the initial breach of the site.

Consistent seasonal variability and site preferences was only apparent for eels which were considerably more abundant in the summer and inside the site. These differences are probably linked to the migratory behaviour of the species. The increase in the number of eels in the realignment may indicate an improvement in habitat quality with increasing time since the site was breached. This observation is a positive note given the current decline of eel populations across Europe. The dominant size spectra in the fykes however, suggests that the main role of the realignment site is as a nursery.

Common prawn (Palaemon longirostris) was found in large numbers but was practically restricted to the river bank where it was the dominant invertebrate species. This species is able to maintain stable populations across brackish estuarine waters. The most common crustacean inside the site was the grass shrimp, a similar brackish estuarine prawn which is known to favour still waters and those with low current velocities. The crustacean community showed a less abundant but more even community inside the site. Of note was the recording of Chinese mitten crabs, an invasive species originated from the coast of Japan.

The epibenthic trawls assessment targeted fish and shellfish of much smaller size than the fyke nets. The intention was to describe the small fish community and epibenthic food resource associated with the site. The trawl data confirmed 3-spined stickleback as a resident species inside the site and along with common goby, and juveniles of dace, roach and breams, comprise the majority of the small fish assemblage using the site.

The majority of invertebrates found inside the realignment (using a trawl) were mysids (Neomysis integer), species well adapted to the low salinity regime of the site. There was some variation between years but more importantly, the density of these animals was found to be highly seasonal with a marked increase in the summer. The summer coincides with the growing season of all fishes and mysids are likely an important prey for the entire small fish assemblage, shrimps (Order Decapoda, Caridea) and larger fish.