Microalgal Colonization in a Saltmarsh Restoration Scheme

Estuarine, Coastal and Shelf Science (1997) 44, 471–481

Microalgal Colonization in a Saltmarsh Restoration Scheme G. J. C. Underwood Department of Biological Sciences, John Tabor Laboratories, University of Essex, Colchester CO4 3SQ, Essex, U.K. Received 13 October 1995 and accepted in revised form 4 March 1996 Spatial, temporal and successional patterns of biomass and species composition of microalgal assemblages were measured on a saltmarsh restoration site (Blackwater, Essex, U.K.) from December 1992 to July 1995. Classification analysis of the microalgae generated six distinct microbial assemblages (two diatom assemblages, D1 & D2; two mixed composition assemblages, M1 & M2; a green algal assemblage, G; and a cyanobacterial assemblage, C). Each of these assemblages differed significantly in the abundance of living and dead diatoms, cyanobacteria, filamentous bacteria and green algae. Microalgal biomass was highest at the beginning of the study and always significantly (P<0·001) higher on the upper and middle marsh stations than on the lower shore stations. Declines in microalgal biomass over time corresponded to increased macrophyte cover and decreasing organic content on the upper and middle marsh stations. Cyanobacterial mats occurred mainly on a region of compacted sediments with a low ash-free dry weight and water content. The succession from immature cyanobacterial mats dominated by Oscillatoria spp. and Spirulina to mats dominated by Microcoleus chthonoplastes took 3 years. Thirteen of the 67 algal taxa identified during the study showed significant preference of occurrence, either by station, assemblage type or season, or with time. Principal component analysis showed a linear sequence between D1, D2, M1, M2 and G type assemblages, relating to the position on shore, macrophyte cover and microalgal biomass. Cyanobacterial assemblages were independent of this successional sequence. There was a significant relationship between the biomass of D1, D2 and M1 type assemblages and the ability to biostabilize sediments through the production of colloidal carbohydrate exopolymers. Assemblage composition needed a relative abundance of diatoms greater than 50% to possess a significant relationship between exopolymer concentration and biomass. It was concluded that diatom films would be the initial colonizers of deposited sediments and that M and G assemblages would develop with increasing sediment bed height in association with increasing macrophyte cover. Compaction of managed retreat sites during construction would lead to the establishment of cyanobacterial mats and delay the development of a typical saltmarsh macrophyte community. ? 1997 Academic Press Limited Keywords: microalgae; managed retreat; microbial mats; diatoms; exopolymers; biostabilization; salt marsh; Blackwater

Introduction Salt marshes are important habitats occurring above the mid-neap tide level in coastal areas where geography and tidal current velocities allow for the deposition of fine sediment particles. A feature of the characteristic macrophyte and associated microphyte communities on salt marshes is a high annual primary production, making salt marshes a significant carbon source to the surrounding benthic and planktonic communities through leaching and transport of dissolved organic carbon (DOC) and particulate organic carbon (POC) (Turner, 1993). Saltmarsh sediments act as important sites of denitrification, thus attenuating nutrient fluxes within estuaries (King & Nedwell, 1985), and act as a physical buffer between land and sea, dissipating the energy of waves and storms (Pethick, 1992). 0272–7714/97/040471+11 $25.00/0/ec960138

Normally, processes of erosion and accretion are balanced over time, and salt marshes and adjacent mud flats exist in an equilibrium set by the local tidalor storm-driven forces (Pethick, 1992). However, relative rises in sea level of 4–5 mm year "1 (due to both natural and anthropogenic factors) have resulted in loss of up to 23% of saltmarsh area in parts of the south-east of England between 1973 and 1988 (Pethick, 1993). This loss has been exacerbated by the presence of defensive sea walls behind many marshes, which prevent landward migration of marsh systems. The re-establishment of salt marshes in areas threatened with coastal erosion is being encouraged by U.K. Government Organizations (NRA, 1991). This policy termed set back or managed retreat involves replacing an existing sea wall at the landward side of an eroded salt marsh with a smaller wall further inland. Salt marsh can then develop in the area in ? 1997 Academic Press Limited

472 G. J. C. Underwood

front of the new sea wall. A number of managed retreat schemes are currently underway or actively planned in Britain. The perceived benefits of managed retreat are low cost and the creation of a natural (and otherwise disappearing) habitat of high conservation status. The policy of managed retreat has been adopted despite reservations that retreat schemes may increase erosion of other sites within the estuarine system (Pethick, 1993). The management of coastal retreat projects and prediction of their outcomes require an understanding of the processes involved in the early seral stages of saltmarsh formation. Microalgae are thought to play an important facilitatory role in the colonization of intertidal sediments by vascular plants (Coles, 1979; Little et al., 1992). Microalgal mats can increase the critical shear stress needed to erode intertidal sediments, by binding sediment particles together with filamentous growth forms, and through the production of extracellular polymeric substances (EPS) (Underwood & Paterson, 1993b; Paterson, 1994; Yallop et al., 1994). The relative contribution of these different mechanisms to increased sediment stability varies with the prevalent physico-chemical conditions and the taxonomic composition of the microalgal assemblages (Paterson, 1994; Underwood et al., 1995), and is therefore dependent on seasonal and successional changes in assemblage composition and biomass (Underwood & Paterson, 1993b; Underwood, 1994). In contrast to the extensive information on successional patterns and processes within vascular saltmarsh plant communities (Gray, 1992), information on microalgal succession during saltmarsh development is scarce. There are numerous studies on diatom assemblage diversity and distribution within saltmarsh habitats (Whiting & McIntire, 1985; Oppenheim, 1991; Laird & Edgar, 1992; Nelson & Kashima, 1993). However, the focus of these studies has not been the successional development of microalgal assemblages during saltmarsh development, and there is no information concerning the long-term development of microalgal assemblages in a developing saltmarsh habitat. Managed retreat sites provide an ideal opportunity to study microalgal colonization processes and the potential for biostabilization. Knowledge of seasonal and successional patterns of microalgal colonization could be useful in determining the optimal timing to promote the establishment of biostabilizing microalgal assemblages, which can then facilitate saltmarsh development. This study recorded the development of microalgal biofilms on the first U.K. managed retreat site, at Northey Island, Essex, U.K., from December 1992 to February 1995, and discusses the importance

(a)

N

Northey Island

51°40'N

10 km

1°E Thames (b)

50 m

1 2 3

4 5

Old sea wall

F 1. (a) The location of Northey Island in the Blackwater Estuary, Essex, U.K. (b) The managed retreat site, showing the position of the five stations used in this study. Stations: 1, upper marsh; 2, middle marsh; 3, lower marsh-wet; 4, lower marsh-mat; 5, upper mud flat.

of these assemblages in the development of new salt marsh. Methods Northey Island (140 ha) is situated in the Blackwater Estuary, Essex, U.K. In July 1991, removal of part of the existing sea wall allowed an area of approximately 0·8 ha to be flooded by the sea (Figure 1). The majority of the retreat site is covered by approximately 100 tides year "1 (Institute of Estuarine and Coastal Studies, unpubl.). The site was disturbed by additional engineering works in May 1992, since when it has been undisturbed. The present study started in December 1992.

Microalgal colonization of a developing salt marsh 473

Five stations, at different tidal heights, were established across the managed retreat site, defined as the upper, middle and low marsh-mat, and low marsh-wet and mudflat stations (Figure 1). Sampling took place between December 1992 and July 1995. All sampling took place during a 3-h period centred around low tide. At each station, sediment surface temperature and salinity (using the practical salinity scale) were measured, macrophyte percentage cover was assessed by eye and five random paired samples of the surface 5 mm of sediment were taken using minicores (cut off 16 mm diameter plastic syringes). One sample from each pair was used for measurement of water content (105 )C for 24 h) and ash-free dry weight (AFDW) (550 )C for 1 h). The second sample in each pair was frozen at "20 )C and lyophilized. Subsamples of this material were then measured for photopigments (Chl a and phaeopigments, Jensen, 1978), and sediment carbohydrate fractions (n=5 for each station). Total carbohydrate concentrations of sediments were measured directly in samples that had not been subjected to any extraction procedure, and therefore all carbohydrate within the range of the assay, including intracellular, extracellular and particle-bound material, was hydrolysed and measured. Colloidal carbohydrate concentrations were measured in a 2-ml sample of supernatant obtained from sediment after 15 min extraction in 25‰ saline (colloidalS), followed by centrifugation at 3620 g for 15 min (see Underwood et al., 1995 for details). In August 1994, minicores 50 mm deep were taken at the upper marsh, low marsh-mat and mudflat stations (n=5 at each station). These cores were frozen, sectioned at 5, 10, 15, 25, 35 and 45 mm depths, and analysed for AFDW and Chl a (methods above). Microalgal assemblages were sampled from four areas of surface sediment at each station. This sediment material was placed in Petri dishes and the microalgal assemblage was harvested over the following 24-h period using repeated (twice) lens tissue sampling (Eaton & Moss, 1966). Subsamples of remaining sediment were also inspected microscopically to ensure no cells were remaining. The harvested microbial material from each station was pooled, fixed in 0·5% w/v glutaraldehyde, and the relative abundance of living diatoms, empty valves (scores÷2 to ensure equal weighting with living diatoms), cyanobacteria, large bacterial filaments and green algae were measured. Filamentous forms were counted in units of 10 ìm lengths. These data were arcsine transformed, and analysed using cluster analysis and principal component analysis. Patterns in the occurrence of different assemblages were determined using

Chi-squared test. Subsamples of fixed material were acid cleaned and permanent mounts were made in Naphrax for further diatom identification and counts of relative abundance based on a sample of 200 valves slide "1. Valves were scored as abundant >40%; common 20–40%, frequent 10–20%, and occasional <10%. The data for individual species of microalgae were analysed with regard to time, station and assemblage type using the Kruskall Wallis k-sample test. Results Microalgal assemblage classification Classification by cluster analysis followed by crossvalidating discriminant analysis of the algal assemblage composition data generated six different microbial assemblages, separated at the 70% level of similarity (Table 1). Each assemblage type had a significantly different mean percentage composition (F5,49 >8·0, P<0·001) in at least one of the five categories measured in the samples (Table 1). These assemblage types were used in further analysis of patterns of microalgal distribution (see below). Physical and biological changes Upper and middle marsh stations. The upper and middle marsh sediments were characterized by high AFDW values and total carbohydrate concentrations (Table 2). Very high AFDW (75·4%) and total carbohydrate (157·6 mg g "1) values at the beginning of the study (due to the presence of decomposing terrestrial plant material) declined with time (AFDW—upper site, F10,44 =7·06, P<0·001; total carbohydrate, upper—F10,42 =24·3, middle—F9,40 = 5·61, P<0·001 in both cases). A layer of spongy and peaty material facilitated a high water content at these two stations, even during hot summer periods. The salinity range was large (16–210) with low values during wetter winter months and high values during periods of hot summer weather (Table 2). Microbenthic Chl a concentrations were initially high but declined significantly at both stations between December 1992 and July 1995. (F10,43 =4·8 for upper, F10,44 =5·05 for middle, P<0·001 in both cases). However, microalgal biomass was generally higher during the winter months. Declines in microalgal biomass over time corresponded to an increase in vascular macrophyte cover (Salicornia sp. and Sueda sp. in 1993, plus Halimone sp. in 1994 and 1995 at the upper station). Microbial assemblage types were variable, with C, D1, D2, M1 and M2 found, but D-type assemblages only occurred when there was noticeable sediment deposition (Figures 2 and 3). Chlorophyll a

474 G. J. C. Underwood T 1. The mean percentage composition (back-transformed from arcsin transformed data) of the six microbial assemblage types found on the managed retreat site

Assemblage Cyanobacterial mat n=16 Diatom biofilm 1 n=16 Diatom biofilm 2 n=3 Mixed 1 n=12 Mixed 2 n=4 Green algae n=2

Code

Diatoms

C

D2

30 `D2 89 _D2 59

M1

50

M2

25 `D2 27

D1

G

Empty valves

Cyanobacterial filament

Bacterial filaments

Green algae

9·5

44 _all 0·5

7·8 _D1,D2 0·8

1·1 0·4

0

0·2

0·3 5·7

0·8

7·0 _D1,D2 2·6

2·2

2·3

64 _all

7·0 40 _C,D1,G 26 _C,D1,G 62 _D2 2·0

4·6

5·1

Percentage composition values significantly (P<0·001) larger (_) or smaller (`) than those of other assemblage types are indicated (thus `D2 indicates that the value is significantly less than the comparable value for D2).

T 2. Median, maximum and minimum temperature, water content, ash-free dry weight (AFDW) and total sediment carbohydrate values for five stations at the Northey Island managed retreat site, December 1992–July 1995 Water content (%)

Station Upper Middle Low-mat Low-wet Mud flat

Median Min. Max. Median Min. Max. Median Min. Max. Median Min. Max. Median Min. Max.

71·5 43·6 85·7 72·5 47·7 90·3 40·2 11·0 69·4 68·6 41·8 81·4 69·2 56·1 95·7

Salinity 33·5 16 210 33·3 19 175 37·5 18 104 32·0 18 70 32·5 22 39

and phaeopigment concentrations on both stations were significantly higher than on the lower-wet or mudflat stations throughout the study (F40,208 =3·23, P<0·001). Low marsh-mat station. The low-mat station was situated on the infilled site of the borrow dyke of the old sea wall. The underlying sediments were compacted and hard, with significantly lower water content (F4,260 =76·3, P<0·001) than the other four stations (Table 2). High initial sediment Chl a concentrations

AFDW (%)

Total carbohydrate (mg g "1 sediment)

30·8 10·7 49·8 22·3 15·5 75·4 13·3 6·7 20·6 13·8 7·6 49·5 10·4 5·1 15·6

75·9 23·9 157·6 56·9 15·1 107·4 21·2 26·8 73·3 19·5 3·2 45·1 7·8 3·9 29·6

(470 ìg g "1 in December 1992) declined during the first 6 months of this study, and then remained fairly constant between 80 and 150 ìg Chl a g "1 (Figure 4). Colonization by Salicornia was slow, reaching a maximum of 30% cover by February 1995. The microalgal assemblages were predominantly cyanobacterial mats (C), but with a D1 type assemblage occurring in February 1993. The cyanobacterial mats were initially composed of Oscillatoria limosa, O. princeps and Spirulina, but Microcoleus chthonoplastes occurred for the first time in August 1994, and

Microalgal colonization of a developing salt marsh 475 T 3. Microalgae showing statistically significant preferences (Kruskall–Wallis P<0·05 *, P<0·01 **, P<0·001 ***) for either station, assemblage type, month of year or period of time on the Northey Island managed retreat site between December 1992 and July 1995 Species

Station

Assemblage

Oscillatoria princeps O. limosa Spirulina Microcoleus chthonoplastes Euglena deses Navicula pargemina N. rostellata N. cincta N. gregaria Pleurosigma angulatum Cylindrotheca closterium Nitzschia frustulum N. panduriformis

LM*** LM*** LM* LM** LW**

C*** C***

Annual

Time

1995**

Upp* Mid.* Mudf.** LW, Mudf.**

February* December–March*

1995*

March, May*

1992–1993* 1993–1994**

D1, D2**

Mudf.*

Upp, upper marsh; Mid, middle marsh; LM, low marsh-mat; LW, low marsh-wet; Mudf., mudflat stations.

400 300

C

+5 mm M1 D1

90 80

C

D2 M1

100

M2

70 60 50 40

200

30 20

100

10 0

D F A J A O D F A J A O D F A J A 1993 1994 1995

0

F 2. Photopigment [chlorophyll a ( ) and phaeopigments ( )] concentrations (means&SE, n=5), percentage cover of vascular macrophytes (hatched bars), and microbial assemblage type (see Table 1 for explanation) at the upper marsh station between December 1992 and July 1995. Arrows indicate recorded deposition events (depth in mm).

was the dominant cyanobacterium by July 1995 (Table 3). Low marsh-wet station. Sediment at the low-wet station had lower AFDW (F4,260 =119·5, P<0·001) and total carbohydrate concentrations (F4,247 =61·1, P<0·001) than the upper or middle stations (Table 2), and lower photopigment concentrations (F40,208 =3·23, P<0·001) than the upper, middle or lower-mat stations. This site was situated along the line of a previous saltmarsh creek, before the marsh had been

500 400 300 200

+3 mm +8 mm

100

+5 mm D1 M1

90

M1 C

C

G D1 C

M1

M1 M2

80 70 60 50 40 30 20

100

% cover macrophytes

500

M1

µg photopigments g–1 sediment

600

M2 D1 M1

600

% cover macrophytes

µg photopigments g–1 sediment

+3 mm +5 mm

10 0

D F A J A O D F A J A O D F A J A 1993 1994 1995

0

F 3. Photopigment [chlorophyll a ( ) and phaeopigments ( )] concentrations (means&SE, n=5), percentage cover of vascular macrophytes (hatched bars), and microbial assemblage at the middle marsh station between December 1992 and July 1995. Arrows indicate recorded deposition events (depth in mm).

reclaimed in the 1770s. Open water became increasingly common on this site over the course of the study as this creek re-developed, and macrophyte colonization was minimal (Figure 5). Sediment Chl a concentrations declined slightly during the study (by ~30%, F10,38 =14·2, P<0·001), with C, D1, M1 and G-type microalgal assemblages occurring (Figure 5). Mudflat station. The lowest sediment AFDW and total carbohydrate values, and the smallest range of salinity and water content values, were recorded from the mudflat station (Table 2). Chlorophyll a concentrations were between 0 and 130 ìg g "1 (Figure 6),

476 G. J. C. Underwood

80 +5 mm

+3 mm +4 mm

70 60

300 200

D1 C

50

C C C

C

C C

40

C

C

30 20

100

10 0

D F A J A O D F A J A O D F A J A 1993 1994 1995

0

500 400 300

100 90

+5 mm G M1

80 70 +3 mm +8 mm

50

C M1

200

60

D1 D1

100

40

M1 M1 M1

D1

D1

30 20

% cover macrophytes

500 400

600

90

µg photopigments g–1 sediment

100

C

% cover macrophytes

µg photopigments g–1 sediment

600

10 0

D F A J A O D F A J A O D F A J A 1993 1994 1995

0

F 4. Photopigment [chlorophyll a ( ) and phaeopigments ( )] concentrations (means&SE, n=5), percentage cover of vascular macrophytes (hatched bars), and microbial assemblage at the low marsh-mat station between December 1992 and July 1995. Arrows indicate recorded deposition events (depth in mm).

F 5. Photopigment [chlorophyll a ( ) and phaeopigments ( )] concentrations (means&SE, n=5), percentage cover of vascular macrophytes (hatched bars), and microbial assemblage at the low marsh-wet station between December 1992 and July 1995. Arrows indicate recorded deposition events (depth in mm).

and were significantly lower than on the upper, middle and lower-mat stations (F40,208 =3·23, P<0·001). There was a sparse growth of Enteromorpha sp. during September 1994, but otherwise no larger plants occurred (Figure 6). The microalgal assemblages were predominantly D-type, with M-type assemblages recorded twice during 1994.

over the 50-mm profile at both the upper marsh and low marsh-mat stations (F6,34 =8·0 and 21·8, respectively, P<0·001 in both cases). There were no significant differences in the mudflat AFDW profile. Chlorophyll a concentrations decreased significantly with depth at all three stations (F6,34 >16·6, P<0·001 in all cases), but Chl a was virtually absent at the low marsh-mat station below 15 mm, while present throughout the profile at the other two stations.

Profiles of AFDW and Chl á with depth. Sediment ADFW was significantly higher at the upper marsh station than at the low marsh-mat or mudflat stations in August 1994 (F2,105 =191·5, P<0·001, Figure 7). The upper marsh ADFW profile peaked at 15–20 mm depth (43·1%) and AFDW decreased significantly

Patterns in assemblage type There were significant patterns in the frequency of occurrence of different assemblage types over the

T 4. Percentage of the total variance in the microalgal assemblage data explained by principal components 1–3, and correlation values between principal components 1–3 and the relative abundance (RA) of different components of the microalgal assemblages and with environmental data (r values, n=55, P<0·05 *, 0·01 **, 0·001 ***)

Variance explained RA diatoms Empty frustules Cyanobacteria Filamentous bacteria Green algae Water content Macrophyte cover Chl a Phaeophytin Site

PC1

PC2

PC3

58·2% "0·928*** n.s. 0·894*** 0·707*** n.s. "0·342** 0·392** 0·313* 0·277* "0·342**

26·6% "0·359** 0·914*** "0·418**

11·4% n.s. "0·390** n.s. n.s. 0·853*** "0·159 "0·120 0·184 0·055 0·054

0·458*** 0·182 0·195 "0·031 0·077 "0·145

Microalgal colonization of a developing salt marsh 477 Log (n + 1) µg Chl. a g–1 sediment

100

1

90 80 70

400

60 300

50 D1

200 100 0

40

D2

D1 D1 D1

30

D1 M1 D2 M2

D1

D1 20 10

D F A J A O D F A J A O D F A J A 1993 1994 1995

0

3

10

F 6. Photopigment [chlorophyll a( ) and phaeopigments ( )] concentrations (means&SE, n=5), percentage cover of vascular macrophytes (hatched bars), and microbial assemblage at the upper mudflat station between December 1992 and July 1995. Arrows indicate recorded deposition events (depth in mm).

20

30

40

50

0

10

20 30 % AFDW

40

50

F 7. Depth profiles of percentage ash-free dry weight and chlorophyll a at three stations during August 1994. Bars (% AFDW) =mean+SE, points (Chl a) =mean&SE, n=5 in all cases. Hatched bars, , upper marsh; stippled bars, 5, low marsh-mat; open bars, , upper mud flat.

0.50

Macrophytes

Water

0.25 PC 2

calendar year, with C and D1 assemblages more abundant during the summer, and M1 being more abundant during the winter (÷2 =24·6, d.f.=14, P<0·05). There were also significant differences in the frequency of occurrence of assemblages on the different stations, with C-type assemblages found at the lower mat station, D1-type assemblages on the lowerwet marsh station and on the upper mud flat, and M1type assemblages on the upper and middle marsh stations (÷2 =29·9, d.f.=8, P<0·001). The first three principal components (PC 1–3) obtained from the principal component analysis explained 96·2% of the variance in the assemblage composition data (Table 4). The major axis of variation (PC 1) was significantly negatively correlated with the relative abundance of living diatoms, and positively correlated with the relative abundances of cyanobacterial and bacterial filaments. PC 2 was significantly negatively correlated with diatom and cyanobacterial relative abundances, and positively correlated with empty valves and green algae, while PC 3 was negatively correlated with empty valves, and positively correlated with green algae and bacterial filaments (Table 4). PC 1 was significantly correlated with sediment water content, macrophyte cover, Chl a and phaeopigment concentration and site position (Table 4). A scatter plot of samples scores of PC 1 and 2 (coded by assemblage type) and the direction and strength of correlation between PC 1 and 2 and environmental variables (Figure 8), showed a significant linear relationship between the D1, D2, M1, M2 and G assemblages (r2 =0·56, n=39, P<0·001), along a gradient related to station (= tidal height),

2

0

Depth (mm)

500

% cover macrophytes

µg photopigments g–1 sediment

600

Phaeo. Chl. A

Station

0.00

–0.25 –0.50 –1.0

–0.5

0.0 PC 1

0.5

1.0

F 8. Scatter plot of principal components (PC) 1 and 2, with samples identified by their assemblage types. Vector plot shows the correlation scores of five variables that had significant correlations with PC1 and PC2 (Table 4). Microalgal assemblages: , D1; , D2; :, M1; , M2; , G; , C.

macrophyte cover and sediment phytopigment concentration. Cyanobacteria-dominated assemblages occurred in a separate region of the plot, separated

478 G. J. C. Underwood

from the other assemblage types along a gradient related primarily to sediment water content. There were no significant differences in the mean number of species of diatoms (mean 12·9&3·7 taxa 200 valves "1) found on the five stations, or in the six different assemblage types. Many diatom species [e.g. Navicula phyllepta (the most abundant and frequent diatom) N. flanatica, N. salinicola, Amphora salina, A. marina] occurred regularly throughout the period of study and showed no significant patterns of abundance. Significant patterns in either station preference, occurrence in specific assemblage type, month of the year when most frequent, or period of time during the study were found for 13 of the 67 taxa recorded (Table 3). Relationship between microalgal biomass and colloidal carbohydrate Sediment Chl a and sediment colloidal carbohydrate concentrations were significantly correlated (P<0·001) in sediments inhabited by D1, D2 and M1-type microalgal assemblages [Figure 9, r=0·65 for D1+D2 and M1 assemblages], but not in samples inhabited by C, M2 and G-type assemblages. The relationship between Chl a and colloidal carbohydrate concentrations was a property of microalgal assemblage type rather than sampling station, as data sorted according to station showed no significant correlation between pigment and colloidal carbohydrate.

Discussion Distribution and seasonal changes in biomass Concentrations of sediment Chl a recorded on the marsh surface in this study were in the upper part of the range of values found in intertidal flats in other studies (Underwood & Paterson, 1993a,b; Yallop et al., 1994). The greater microalgal biomass on the upper compared to the lower areas of the intertidal zone agree with previous measurements in the intertidal (Colijn & Dijkema, 1981; Underwood & Paterson, 1993a). Greater algal biomass on upper shores is probably due to the contributory effects of reduced tidal disturbance and longer emersion period permitting primary production (Underwood & Paterson, 1993b). These benefits are balanced against the increased potential for desiccation with increasing height on the shore, particularly during the warmer months, resulting in a monomodal distribution of microalgal biomass across the tidal range.

Microalgal biomass in salt marshes is usually as high as, or higher than, on adjacent mud flats (Oppenheim, 1991; Pinckney & Zingmark, 1993; Brotas et al., 1995), and this has been attributed to protection from re-suspension etc. by the presence of macrophytes (Oppenheim, 1991). Macrophytes were absent at the start of this study, but increased during the 3 years. Increases in macrophyte cover corresponded to a decline in benthic Chl a concentrations on the marsh surface and a shift of microalgal assemblage types from D1 through to M2 (Figure 8). Although dense stands of macrophytes shade the marsh surface, a degree of shading in itself is not deleterious (Sullivan & Moncreiff, 1988). Epipelic diatoms appear to be shade-adapted and assemblages growing in dense Spartina stands are more efficient, reaching their maximum rate of production (Pmax) at lower irradiances than open mudflat assemblages (Sullivan & Moncreiff, 1988; Pinckney & Zingmark, 1993). Indeed, under very high light intensities (1900 ìE m "2 s "1), diatoms actually migrate down into the sediment (Miles et al., unpubl. obs.). Therefore, increasing macrophyte cover on the retreat site may not be causative of the decline in microalgal biomass. Microalgal biomass was significantly higher on the areas of the site where decomposing plant material was present. These stations initially had a high sediment organic content (max. 49·8% AFDW), typical of sediments in irregularly flooded salt marshes (Craft et al., 1993). Profiles of sediment AFDW from the upper marsh showed a high organic content down to 50 mm, and the development of a peaty layer which retained water during the summer periods. This may have enhanced the development of benthic microalgal biofilms during the early stages of the site due to the reduction of desiccation pressure and increased inorganic nutrient flux from the anaerobic decomposing layers (particularly NH4 + as a product of nitrate ammonification, due to the high C:N ratio, King & Nedwell, 1985, 1987). Declines in algal biomass may have been due to the reduction in the organic content and water retentive capacity of the underlying sediments (Table 2) as decomposition proceeded and mineral sediment was deposited and incorporated. Lower AFDW values and Chl a concentrations in profiles suggest that deposition of organic and mineral material has occurred on the upper marsh, possibly to a depth of 10–15 mm over the 3-year period of study (Figure 7). Such values agree with measured rates of accretion of 2·1–5·4 mm year "1 on an irregularly flooded salt marsh in the U.S.A., due to a combination of reduced tidal flushing, mineral deposition and retention of organic material (Craft et al., 1993).

Microalgal colonization of a developing salt marsh 479

Successional changes in assemblage type and algal species µg colls carbohydrate g–1 sediment

Multivariate analysis showed that distinct microalgal assemblage types occurred, and that these assemblages could be considered as two different systems (Figure 8). With increasing height of the station (and of the marsh as a whole as deposition of sediment occurs) and with the development of a vascular plant community, microalgal assemblages of the M1, M2 and G-types will become increasingly abundant. Superimposed over this successional pattern are the seasonal patterns of M1 assemblages being more likely to occur during the winter, and D1 and C being more likely to occur during the summer. Separate from this sequence were C-type assemblages, which occurred on dry and compacted sediments (Figure 8, Table 4). Laird and Edgar (1992) and Nelson and Kashima (1993) both showed changes in diatom assemblages with increasing height or distance from the sea. These changes in composition were gradual, and the assemblages were often defined by the presence of a number of characteristic species. Similar patterns were present at Northey, with a number of taxa showing preference for particular stations, assemblage types or seasons (Table 3). In some cases, preferences agree with the limited synecological information available. Euglena deses appears to prefer high sediment water contents (Round, 1971; Underwood, 1994), and was found preferentially on the low organic content, wet sediments of the low marsh-wet station. Cylindrotheca closterium was more abundant during the spring (Kawamura & Hirano, 1992; Underwood, 1994) and N. pargemina occurred more during the winter than summer (Underwood, 1994). As has been found in studies of intertidal diatom assemblages (Whiting & McIntire, 1985; Underwood, 1994), other important taxa (e.g. N. phyllepta) showed no patterns, being abundant throughout the study on all the stations. Although C-type assemblages did occur on all four marsh stations during the study, they were dominant at the low-mat station, where the sediments had been compacted. Studies on intertidal cyanobacterial mat development have found that Oscillatoria spp. and Spirulina are early seral stage taxa, indicative of a lownutrient environment (Stal et al., 1985), with Microcoleus chthonoplastes occurring in mature mats (Esteve et al., 1992). The ability of Oscillatoria spp. to fix nitrogen (Stal et al., 1985) may give these taxa an advantage on the low water content, low AFDW sediments of the compacted region. The absence of M. chthonoplastes in the C-type assemblages until August 1994, and its dominance in 1995 (Table 3)

7000 6000

r = 0.65

5000 4000 3000 r = 0.65 2000 1000 0

0

100 200 300 µg chlorophyll a g–1 sediment

400

F 9. The relationship between sediment chlorophyll a and colloidalS carbohydrate concentrations for D1 ( ), D2 ( ) and M1 (:) assemblages.

indicates that this succession to mature mats takes at least 3 years. The mature mats appear to be able to tolerate a drier and more saline environment, and are not part of the normal succession of microbial assemblages on the site. Biostabilization potential The ability of microalgal biofilms to increase the critical erosion threshold of intertidal sediments has been demonstrated in a number of studies (Paterson et al., 1990; Underwood & Paterson 1993a,b; Madsen et al., 1993; Paterson, 1994). Two mechanisms of biostabilization are the formation of a physical network and the production of extracellular polymeric substances (EPS). Measurements of colloidal carbohydrate in sediments give an index of the quantity of EPS present (Underwood et al., 1995), and for diatom-rich assemblages, there is usually a linear relationship between biomass and colloidal carbohydrate concentration (Underwood & Paterson, 1993a,b; Underwood et al., 1995). This relationship was also found for the D1, D2 and M1 assemblages in this study (Figure 9). These three assemblage types consisted of >50% of living epipelic diatoms, the algae mainly responsible for producing colloidal carbohydrate in sediments (Edgar & Pickett-Heaps, 1984; Madsen et al., 1993). In diatom-rich assemblages, EPS is approximately 20% of the material in a colloidalS extract (Underwood et al., 1995). Therefore, the potential of these assemblages to biostabilize sediments through the production of EPS can be directly related to their biomass. The C, M2 and G assemblages, in which the abundance of diatoms was <30%, did not show a

480 G. J. C. Underwood

good relationship between Chl a and colloidal carbohydrate. Cyanobacteria and filamentous bacteria do produce EPS, but this material is more tightly bound to the cells than colloidal EPS (Underwood et al., 1995). Cyanobacterial mats can also stabilize sediments by physical binding of sediment particles, and the development of a smooth layer of bound filaments over the surface of the sediment (Paterson, 1994). Compared to both the upper marsh and mudflat stations, the cyanobacterial mats at the lower marshmat station showed far greater stratification of Chl a (Figure 7), indicating less mixing than at the other sites. The mudflat profiles (of both AFDW and Chl a) suggest almost complete mixing down to at least 50 mm. In addition to stabilizing existing sediments, biofilms are involved in marsh accretion processes, trapping newly-deposited sediment before it is removed by the next tide. Diatom films have been shown to increase the retention of freshly-deposited sediments by producing EPS as they move through the material to the surface (Coles, 1979; Underwood & Paterson, 1993a). D1-type assemblages were common when there had been obvious sediment-deposition events (measurable depths of up to 5 mm), and the presence of such assemblages will have increased the critical erosion threshold of such material. Under favourable conditions, epipelic diatom biomass can have a daily doubling rate (Underwood & Paterson, 1993a), and the combination of rapid motility and population growth rate enables diatom films to be efficient stabilizers of freshly-deposited sediments. Observations (unpubl.) of cyanobacterial mats suggest that migration of the filaments to the surface was a much slower process than that for diatom films, and the presence of C-type assemblages on the firmer, less mixed station supports the hypothesis that these assemblages cannot develop in dynamic sediment situations. Therefore, although C-type assemblages may be able to stabilize sediments, their ability to trap fresh material would appear to be less than that of the more dynamic D1 or M1-type assemblages.

Importance of mats as precursors of marsh development As there are no methods for experimentally removing the effect of biostabilization by microalgae without significantly changing the other processes occurring on a site, it is impossible to show that the absence of mats would prevent saltmarsh development. However, given our knowledge of the processes of biostabilization and the occurrence of different types of assemblages at different tidal levels or times of the

year, it is possible to make some predictions concerning microalgal mat succession in developing marsh sites. The type of microalgal mats that will develop in a managed retreat site will depend on the height of site within the tidal prism, the nature of the underlying sediments (organic content, compaction), and the time of year. Low sites will predominantly support D-type assemblages. With increasing marsh height, there is a gradient of assemblage types, through to mixed assemblages of algae. Biostabilization by these assemblages would act synergistically with physical deposition processes to promote sediment accretion. Initial recommendations for management of sites included rotavation and rolling to reduce vascular plant debris and produce a consolidated substratum for macrophyte colonization. However, excessively compacted sites are likely to develop cyanobacterial mats and have low colonization rates by vascular macrophytes, while retention of decaying terrestrial vegetation on sites encourages both high biomass, mixed assemblages mats to develop, and macrophyte colonization. Biofilm-mediated biostabilization during saltmarsh regeneration would be maximal in the early months of a site, and decline over time as terrestriallyoriginated organic matter declined and macrophyte cover increased. Acknowledgements The author thanks Dr J. Dagley at English Nature and Mr K. Turner at the National Trust for helpful discussions, and permission to carry out work on the Northey Island managed retreat site. Dr M. L. Yallop is also thanked for useful discussion of this work. References Brotas, V., Cabrita, T., Portugal, A., Seroˆdio, J. & Catarino, F. 1995 Spatio-temporal distribution of the microphytobenthic biomass in the intertidal flats of Tagus Estuary (Portugal). Hydrobiologia 300/301, 90–104. Coles, S. M. 1979 Benthic microalgal populations on intertidal sediments and their role as precursors to salt marsh development. In Ecological Processes in Coastal Environments: The First European Symposium of the British Ecological Society (Jefferies, R. L. & Davy, A. J., eds). Oxford, Blackwell Scientific, Oxford, pp. 25–42. Colijn, F. & Dijkema, K. S. 1981 Species composition of benthic diatoms and the distribution of Chlorophyll a on an intertidal flat in the Dutch Wadden sea. Marine Ecology Progress Series 4, 9–21. Craft, C. B., Seneca, E. D. & Broome, S. W. 1993 Vertical accretion in microtidal regularly and irregularly flooded estuarine marshes. Estuarine, Coastal and Shelf Science 37, 371–386. Eaton, J. W. & Moss, B. 1966 The estimation of numbers and pigment content in epipelic algal populations. Limnology and Oceanography 11, 584–595. Edgar, L. A. & Pickett-Heaps, J. D. 1984 Diatom locomotion. In Progress in Phycological Research (Round, F. E. & Chapman, D. J., eds). Biopress, Bristol, pp. 47–88.

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Pethick, J. S. 1992 Saltmarsh geomorphology. In Saltmarshes: Morphodynamics, Conservation and Engineering Significance (Allen, J. R. L. & Pye, K., eds). Cambridge University Press, Cambridge, pp. 41–62. Pethick, J. S. 1993 Shoreline adjustments and coastal management: physical and biological processes under accelerated seal-level rise. The Geographical Journal 159, 162–168. Pinckney, J. & Zingmark, R. G. 1993 Photophysiological response of intertidal benthic microalgal communities to in situ light environments: methodological considerations. Limnology and Oceanography 38, 1373–1383. Round, F. E. 1971 Benthic marine diatoms. Oceanography and Marine Biology Annual Review 9, 83–139. Stal, L. J., van Gemerden, H. & Krumbein, W. E. 1985 Structure and development of a benthic marine microbial mat. FEMS Microbiology Ecology 31, 111–125. Sullivan, M. J. & Moncreiff, C. A. 1988 Primary production of edaphic algal communities in a Mississippi salt marsh. Journal of Phycology 24, 49–58. Turner, R. E. 1993 Carbon, nitrogen and phosphorus leaching rates from Spartina alterniflora salt marshes. Marine Ecology Progress Series 92, 135–140. Underwood, G. J. C. & Paterson, D. M. 1993a Recovery of intertidal benthic diatoms from biocide treatment and associated sediment dynamics. Journal of the Marine Biological Association of the U.K. 73, 25–45. Underwood, G. J. C. & Paterson, D. M. 1993b Seasonal changes in diatom biomass, sediment stability and biogenic stabilization in the Severn Estuary. Journal of the Marine Biological Association of the U.K. 73, 871–887. Underwood, G. J. C. 1994 Seasonal and spatial variation in epipelic diatom assemblages in the Severn estuary. Diatom Research 9, 451–472. Underwood, G. J. C., Paterson, D. M. & Parkes, R. J. 1995 The measurement of microbial carbohydrate exopolymers from intertidal sediments. Limnology and Oceanography 40, 1243–1253. Whiting, M. C. & McIntire, C. D. 1985 An investigation of distributional patterns in the diatom flora of Netarts Bay, Oregon, by correspondence analysis. Journal of Phycology 21, 655–661. Yallop, M. L., Winder, B. de, Paterson, D. M. & Stal, L. J. 1994 Comparative structure, primary production and biogenic stabilisation of cohesive and non-cohesive sediments inhabited by microphytobenthos. Estuarine, Coastal and Shelf Science 39, 565– 582.