Structure of a mudflat diatom community in the Avon Heathcote Estuary, New Zealand

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1 New Zealand Journal of Marine and Freshwater Research ISSN: (Print) (Online) Journal homepage: Structure of a mudflat diatom community in the Avon Heathcote Estuary, New Zealand S. McClatchie, S. K. Juniper & G. A. Knox To cite this article: S. McClatchie, S. K. Juniper & G. A. Knox (982) Structure of a mudflat diatom community in the Avon Heathcote Estuary, New Zealand, New Zealand Journal of Marine and Freshwater Research, 6:3-4, , DOI: 0.080/ To link to this article: Published online: 22 Sep 200. Submit your article to this journal Article views: 56 View related articles Citing articles: 5 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 6 September 206, At: 20:07

2 New Zealand Journal of Marine and Freshwater Research, 982, Vol. 6 : Structure of a mudflat diatom community in the Avon-Heathcote Estuary, New Zealand S. McCLATCHIE S. K. JUNIPER G. A. KNOX Department of Zoology University of Canterbury Private Bag Christchurch, New Zealand Abstract Parameters of community structure (species composition and relative abundance, number of taxa, diversity, evenness, and cell density) were measured for a mudflat diatom community in the Avon-Heathcote estuary, New Zealand. Fifty three diatom species were identified: 25 taxa (species and varieties) are new New Zealand records. The Shannon-Wiener information index (H') was 3.46, indicating high diversity. Evenness (J') ranged from The association between the biomass of the pulmonate gastropod, Amphibola crenata, and benthic diatom community structure was studied using large open enclosures (4.0 m 2 ) to manipulate snail biomass. Community structure was compared at 0, 5 (natural biomass), and 0 g A. crenata dry weight per m. A similarity index (S/M7= ), as well as H' and J' indicated close similarity between the diatom assemblages within all enclosures, but number of taxa increased from 33 to 49 with increasing snail biomass. Cell densities were significantly lower at high snail biomass (6088 valves per mm 2 ) compared to enclosures with no snails (0 0 valves per mm ). A. crenata had a higher ratio of diatom fragments to whole diatom valves in its faeces (2.42) than in its crop (0.55), indicating that it is capable of fragmenting diatoms. Keywords benthos; diatoms; Avon-Heathcote Estuary; community composition; new records; Amphibola crenata; detritus feeders; intertidal environment; aquatic communities; estuaries. Received 3 January 982; accepted 9 August 982 Present address: Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 4J Institute of Ocean Sciences, P.O. Box 6000, Sydney, British Columbia, Canada. INTRODUCTION The benthic diatom floras of New Zealand estuaries are poorly known. Most works on New Zealand diatom floras have been broad in coverage and floristic in nature. General floristic studies (Crosby & Wood 959; Wood 963) include species tolerant of brackish water, as does Cassie's (96) survey of the phytoplankton, which lists some tychopelagic species. Where a study has focused on an estuarine system (Stidolph 980) many new records have been added to existing checklists of marine and freshwater diatoms from New Zealand. There are no detailed floristic or experimental studies on the diatom communities in the Avon- Heathcote estuary, Christchurch. Although Knox & Kilner (973) listed 7 genera as either common or uncommon on the estuarine mudflats, and Bennington (97) added several genera to the list, neither attempted to catalogue the species or describe the community structure for any locality within the estuary. This study provides the first data of this kind for locality in the Avon-Heathcote estuary. There have been no ecological studies on the factors governing the structure or distribution of sediment-associated diatoms in New Zealand. Elsewhere, various authors have shown that the structure of edaphic communities is affected by sediment type (Riznyk & Phinney 972), salinity gradients (Sullivan 977), marsh surface temperature, and exposure to desiccation (Sullivan 975, 976; Mclntyre 973). Manipulation experiments have shown that reducing marsh grass cover can initiate a shift in diatom community structure as a result of both increased desiccation and interaction with greatly increased numbers of blue-green algae (Sullivan 976). Long-term nutrient enrichment with nitrogen and phosphorous may stress edaphic diatom communities and reduce their diversity (Sullivan & Darber 975). Both Sullivan (978) and Amspoker & Mclntyre (978) report a very significant portion of unexplained variability (68% and 50% respectively) in their multiple regression analyses of factors affecting community structure. Biological processes, such as grazing, amensalism, competition for substrates, and the physiological tolerances of the diatoms themselves must be important factors contributing to this variability. In recent years experimental manipulations of soft-bottom benthic communities have gained increasing popularity, and have proved to be

3 300 New Zealand Journal of Marine and Freshwater Research, 982, Vol. 6 valuable tools for the study of biological interactions (Woodin 974; Young et al. 976; Riesse 977; Hulberg & Oliver 980). Such experiments have primarily involved the use of cages of varying designs to exclude predators or competitors. The present study was designed to take advantage of a series of enclosures that had been established in the Avon-Heathcote estuary to study the impact of Amphibola crenata on the productivity of the mudflat microbial community (Juniper, unpublished data). METHODS The Avon-Heathcote estuary is located about 0 km from Christchurch, New Zealand (43 33'S, 72 44'W), and receives freshwater input from the Avon and Heathcote rivers and the Bromley oxidation ponds. The study area was located in the upper southeast corner of the estuary, adjacent to the Heathcote channel on an open mudflat at midtide level (8.89 m above Chart Datum). The site was exposed to air for 3-5 hours per tidal cycle, but sediments retained a surface film of water. Salinity ranged from x 0~ 3, with a surface-water ph of approximately 6.0 at low tide. Scattered drift seaweed was present, but marsh grasses and filamentous microalgae were absent. Mud core samples were taken from within and outside enclosures, 2.0 x 2.0 m in size, constructed of 8 mm plastic mesh attached to a metal frame, with no roof, sunk into the mudflat to a depth of 0.5 m (Fig. ). Walls of the enclosures served to retain or exclude Amphibola, so that snail biomass could be manipulated, while allowing relatively free passage of water. Two enclosures excluded Amphibola (Excl-Cages), while 2 more enclosures (N-Cages) were maintained with a snail biomass equivalent to that on the adjacent mudflat (approx. 5 g m~ 2 dry weight). The remaining 2 enclosures (2 x Cages) were loaded with twice this snail biomass (approx. 0 g m 2 dry weight). The enclosures had been in place for month when the mud samples were taken (5 May 98). Cores of the top 0 mm of sediment were obtained at low tide using a 50-mrn-diameter corer. Five cores were taken from each of the Excl-Cages and N-Cages, but 0 cores were taken from the single remaining 2x Cage after the other was destroyed. Five cores were taken for sediment analysis from randomly selected points 50 m apart outside the enclosures. Subsamples of 2.5 ml were measured by volumetric displacement, and cleaned of organic matter using nitric acid (Hohn & Hellerman 963). Cleaned diatoms were separated from sand grains by repeatedly decanting, after allowing 60 seconds for the coarse particles to settle. 05 mj Fig. Controlled Amphibola grazing enclosure (Cage), Avon-Heathcote estuary. To permit identification of cells unobscured by finer sediment particles, cleaned samples were diluted to 250 ml before preparing mounts in Hyrax (refractive index =.63). The permanent slide collection is retained at Dalhousie University. Diatoms were photographed for identification at 000 x and at 250 x using phase-contrast enhancement. To compensate for the tendency of diatoms to concentrate near the centre of the coverslip, cells were counted along transects across the entire coverslip. All counts were made at 250 x until 500 cells had been identified at each snail density, a sample size recommended by Mclntyre & Overton (97). Diatoms were identified primarily by reference to Patrick & Reimer (966), Peragallo & Peragallo (908), Hustedt (930a,b 955), Hendey (964), Lange-Bertalot (979, 980), Giffen (975, 976), and Stidolf (980). Community structure was analysed by number of taxa, the Shannon-Wiener Information Index, and Evenness. Diversity is given by: P i where P t is the proportion of the ith species in the community (Shannon & Weaver 949). This index is more appropriate to a fully censused population (Pielou 966) rather than to the small sample examined here. However, the absolute measure of sample diversity that would be provided by Brillouin's Index (H) cannot be used because of the number of individuals per sample. Evenness was calculated as: where W max = log S, and S = number of species (Pielou 966). A similarity index, SIMI, proposed by Stander (970), and used by Sullivan on diatom communities, was used to compare different enclosures. This index is defined as: SIMI - 2 P H P,, ',?, /,?,

4 McClatchie et al. Mudflat diatom community structure 30 Table Nutrient content of sediments within experimental Cages in the Avon-Heathcote estuary, 5 May 98. (x±s.d., n = 2, except for 2x Cage, where n = ) n represents 0 pooled core samples (each 0 mm diameter). 2 x-cage N-Cage Excl.-Cage % moisture ± (wet wt) % organic content ±0.0 (dry wt) Acid soluble silicate (dry wt mgkg" ) ) Acid soluble phosphorus Total phosphorus (dry wt mg kg" (dry wt mg kg" ) Nitrate-N (mg kg" ) Ammonical-N(mgkg-') ± ± ±0.35 where P t- and P in are the relative abundance (expressed as a proportion) of the ith species in the /th and nth communities respectively, and S is the total number of species. A maximum value of.0 indicates identical species composition and relative abundances, whereas a value of 0 indicates no taxa in common. To assess the impact on diatoms of passage through the gut of Arnphibola, samples of crop contents and faeces from 5 snails freshly collected from the field were acid cleaned and the relative numbers of intact and fragmented diatom valves were compared. Fresh Amphibola faeces were examined for diatoms surviving gut passage, as indicated by intact chloiroplasts. RESULTS Nutrient data for sediments in the enclosures are given in Table. Sediment particle size distribution data are given in Table 2. Core samples from outside and within the enclosures were compared between the particle size limits set by the 5th and 95th percentiles (2-8 <)>). Within this range the cumulative sediment particle size distribution curves were visually indistinguishable inside and outside the enclosures, suggesting a minimum of cage effects on sediments, a persistent problem in many enclosure studies (see Discussion). The mean <j> diameter of 3.50 (0.088 mm) is classified as very fine sand (Inman 952) and we found that no detectable deposition of silt (4 8 4>) occurred within the enclosures. New records Descriptions of 25 taxa which are new records for New Zealand are given below. For convenience, species are recorded in alphabetical order, with Table 2 Sediment particle size distribution parameters for experimental cages in the Avon-Heathcote estuary, (all samples combined), 5 May 98. (Md<t>, phi median diameter; CT4>, phi deviation measure; a<)>, phi skewness measure; oc2<j>, 2nd phi skewness measure; 3<t>, phi kurtosis measure), cjmnit = -log 2 particle diameter in mm. Sediment parameters follow Inman (972). Particle diameter (cj>) Sand <4 Silt Clay Sum Central tendency Sorting Skewness Kurtosis ( ) % of total sources for all identifications. As many as possible of the diatoms identified have been illustrated in Fig. 2, 3, and 4 to permit the authors' identifications to be used by other workers. Habitat type is designated as freshwater (fw), marine (m), or brackish (bw). Achnanth.es conspicua var. brevistriata Hust., Fig. 2/4 (fw). cf. Hustedt (930b), p. 202, fig Dimensions: 8-9 p.m broad. Striae: 2-4 in 0 xm. Achnanthes levanderi Hust., Fig. 2/3 cf. Hustedt (930a), vol. 2, p. 404, fig Dimensions: 7-2 u,m long, 4-5 xm broad. Striae: in 0 xm. Amphora perpusilla Grun., Fig. 2/8, (m-fw). cf. Patrick & Reimer (966), vol. 2, pt., p. 70, pi. 3, fig. 8-. Dimensions: 7 jjim long, 2 u,m broad. Striae: 9-2 in 0 u.m. Cyclotella caspia Grun., Fig. 4/ (bw-m). cf. Hendey (964), p. 74. Dimensions: 6-8 (jum diameter. Striae: 4 6 in 0 \x.m. Diploneis oblongella Naegeli, Fig. 2/, (fw-bw). cf. Patrick & Reimer (966), vol., p. 43, pi. 38, fig- 8. Dimensions: 6-20 xm long, 8-0 u,m broad. Striae: 0-2 in 0 u,.

5 302 New Zealand Journal of Marine and Freshwater Research, 982, Vol. 6 Fig. 2 () Navicula mendotia; (2) JV. confervacea var. peregrina; (3),(4) JV. diserta; (5) N. mutica var. cohnii; (6) Amphora sp. B; (7) Amphora sp. A, 2 attached valves; (8) A. perpusilla; (9),(0) A. coffeaeformis; () Diploneis oblongella, broken valve; (2) Achnanthes wellsiae; (3) A. levanderi; (4) A. conspicua var. brevistriata; (5) A. lanceolate; (6) A. hauckiana; pseudoraphe valve; (7),(8) A. hauckiana, raphe valve; (9) Achnanthes sp. indeterminate; (20) Fragillaria construens var. wmter ; (2),(22) Nitzchia romana; (23) JV. thermalis; (2A) N. ovalis; (25) N. frustulum; (26),(27) JV. inconspicua; (28) JV. angustata; (29) JV. hantzchiana; (30) N. thermalis var. minor; (3),(32) N. fonticola; (33) JV. apiculata; (34) Nitzchia sp. A; (35) Nitzchia (Tryblionella Section).

6 McClatchie et al. Mudflat diatoim community structure 303 Fig - 3 () Gyrosigma peisonis; (2),(3) Synedra fasciculata var. truncata; (4) Plagiogramma appendiculatum; (5) P. (cue; (6),(7) Licmophora sp. A; (8),(9),(0) Opephora parva; () O. martyi; (2) Rhabdonema crassum; (3) Cymbella microcephala; (4) Cocconeis sp. A; (5) C. disculus; (6),(7),(8) C. thumensis; (9),(20) Gomphonema littorale; (2) G. olivaceum var. minwtissima; (22) Amphiprora sp. indeterminate; (23) Navicula luzonensis; (24) N. tenera; (25) Navicula sp. B; (26) N. radiosa.

7 304 New Zealand Journal of Marine and Freshwater Research, 982, Vol. 6 Fig. 4 () Cyclotella caspia; (2) Achnanthes sp. A; (3) Navicula vaucheriae; (4) Achnanthes hauckiana var. rostrata; (5) Navicula sp. A; (6) Grammatophora angulosa; (7) Nitzchia hantzchiana; (8) Synedra fasciculata var. truncata; (9) Navicula radiosa var. parva; (0) Cocconm placentula var. euglypta; () Gyrosigma fasciola. 8 Gomphonema Uttorale Fig. 4/9, 20 (m). cf. Hendey (976). Dimensions: 3-20 (Jim long, 2-3 (xm max. width. Striae: 5-8 in 0 \xm. Gomphonema olivaceum var. minutissima Hust., Fig. 4/2 (bw). cf. Hustedt (930b), p. 378, fig Dimensions: 0 p,m long, 2.5 xm broad. Striae: 7-8 in 0 xm. Gyrosigma peisonis Grun., Fig. 3/, (bw). cf. Hustedt (955), p. 34, pi. 0, fig. 4, 5. Dimensions: 58 jjim long, 0 xm broad. Striae: in 0 xm transapical; in 0 xm longitudinal. Navicula diserta Hust., Fig.2/3,4, (bw-m). Dimensions: 8-9 ^.m long, 3 p.m broad. Striae: 8-20 in 0 (j,m

8 McClatchie et al. Mudflat diatom community structure 305 a b c d e f g h i j k l m n o DIATOM SPECIES 42.4% Fig. 5 Relative abundance of diatom species constituting more than % of the community in Cages, 5 May 98. () Amphora coffeaeformis; (b) Navicula mendotia; (c) Achnanthes wellsiae; (d) Nitzchia frustulum; (e) Navicula diserta; (f) Opephora parva; (g) Nitzchia inconspicua; (h) Navicula spp. indeterminate; (i) Gomphonema littorale; (j) Cyclotella caspia; (k) Achnanthes conspicua var. brevistriata; () Amphora perpusilla; (m) Cocconeis disculus; (n) Nitzchia hantzchiana; (o) species constituting less than % of the community. Navicula mendotia VanLandringham, Fig.2/, (bw) cf. Patrick & Reimer (966), vol., p. 534, pi. 5, fig. 8 (Navicula dulcis). Dimensions: 9-33 u,m long, 5-8 (im broad. Striae: 9- in 0 xm. Navicula luzonensis Hust., Fig. 3/23, (fw). cf. Patrick & Reimer (966), vol., p. 492, pi. 46, fig. 24. Dimensions: 7-8 u,m long, 4 (im broad. Striae: 8-20 in 0 u.m Navicula mutica var. cohnii (Hilse) Grun., Fig. 2/5 (m-bw). cf. Hustedt (930a), vol. 3, p. 583, fig. 592 Dimensions: 2 jim long, 6 (xm broad. Striae: 20 in 0 ujn. Navicula tenera Hust., Fig. 3/24. cf. Patrick & Reimer (966), vol., p. 44, pi. 39, fig. 2 Dimensions: 2-6 xm long, 5-6 UJH broad. Striae: 2-8 in 0 nm. Nitzchia fonticola Grun., Fig. 2/3,32, (fw) cf. Lange-Bertalot (977), p. 270, fig. 3, no. 7-. Dimensions: 30 xm long, 6 urn broad. Striae: in 0 xm. Fibulae: 3 in 0 u.m. Nitzchia frustulum (Kutz) Grun., Fig. 2/25 cf. Hustedt (930b), p. 44, fig. 795 Dimensions: 6-9 (Jim long, 2-3 fxm broad. Striae: in 0 jxm. Fibulae: -3 in 0 u.m. Nitzchia inconspicua Grun., Fig. 2/26, 27, (bwm). cf. Lange-Bertalot (977), p. 265, fig. 2, no Dimensions: 7-9 xm long, 2 u,m broad. Striae: in 0 xm. Fibulae: 0-4 in 0 (xm. Nitzchia ovalis Arnott, Fig. 2/24, (m). cf. Hustedt (930b), p. 47, fig Dimensions: 0 \xm long, 4 xm broad. Striae: not visible. Fibulae: 7 in 0 (xm. Nitzchia romana Grun., Fig. 2/2, 22, (fw). cf. Hustedt (930b), p. 44, fig Dimensions: 7 jxm long, 4 xm broad. Striae: in 0 u.m. Fibulae: 2-4 in 0 ujn. Nitzchia thermalis var. minor Hilse, Fig.2/30, (fw). cf. Lange-Bertalot (978), p. 642 (Nitzchia hombergiensis). Dimensions: 7 (xm long, 5 jxm broad. Striae: in 0 jxm. Fibulae: 8-9 in 0 u.m. Nitzchia sp. (Triblionella Section), Fig. 2/35. Dimensions: 25 xm long, 2 jxm broad. Striae: 7-8 in 0 u j n. Opephora parva (Grun.) Krasske, Fig. 3/8,9,0, (m). cf. Giffen (976), p. 39, fig Dimensions: -3 u.m long, 3-4 (xm max width. Striae: 0-4 in 0 xm. Plagiogramma cf. appendiculatum Giffen, Fig. 3/4 (m). cf. Giffen (975), p. 9, fig. 09. Dimensions: 42 ^.m long, 2 u,m broad. Striae: 4-5 in 0 xm. Plagiogramma leve (Greg.) Ralfs, Fig. 3/5, (m). cf. Hustedt (930a) vol. 2, p. 2, fig Dimensions: xm long, 8-0 jxm broad. Striae: 6-8 in 0 (xm. Rhabdonema crassum Kutz., Fig. 3/2, (bw). cf. Hendey (964), p. 72, pl.6, fig Dimensions: 9 xm long, 9 xm broad. Striae: 7 in 0 jxm, puncti: 5 in 0 \x.m.

9 306 New Zealand Journal of Marine and Freshwater Research, 982, Vol. 6 Synedra fasciculata var truncata (Grev.) Patrick, fig. 3/2,3, (fw-bw). cf. Patrick & Reimer (966), vol., p. 42, pi.5, fig. 6. Dimensions: 55 u,m long, 5 xr n broad. Striae: -2 in 0 \xm. Community structure Diatoms were identified to species or varietal level wherever possible, but sometimes could only be identified to genus. 30% of all cells counted were impossible to identify because they were resting in girdle view, or because the valves were obscured. The predominant genera were Nitzchia ( species), Navicula (0 species), Achnanthes (6 species), and Amphora (4 species). Sixty-four diatom taxa were identified, 25 of which have not been previously reported from New Zealand. The majority of the species were small pennate diatoms, often less than 20 n-m in length. Very few centric diatoms were found (Cyclotella caspia, Coscinodiscus sp.). Only 4 species occurred commonly enough to constitute more than % of the community (Fig. 5). This assemblage of numerically dominant species collectively comprised 57% of the community. The remaining 43% was made; up of a large number of species represented by few individuals. Assemblages in all enclosures had very similar species composition and relative abundances of taxa, shown by SIMI values between 0.88 and 0.95 (Table 3). Diversity was also similar between enclosures. The number of diatom taxa showed a progressive increase with increasing snail density. Only 33 taxa were found in the Excl-Cages, compared to 40 taxa in the N-Cages, and a markedly higher number of 49 taxa in the 2 x -Cage. Since T was lower in the 2 x -Cage, this increase in the number of taxa can be attributed to relatively rare species. The major difference in community structure between the enclosures appears to be the number of rare species present, all other indices being similar. Since many of the rare species were represented by as few as or 2 valves, the strong similarities between enclosures shown by the other indices justifies combining data from different enclosures to provide an estimate of the relative abundance in the community of the species encountered (Table 4). Diatom cell densities and vulnerability to digestion Preliminary chlorophyll a data showed no difference in algal biomass inside and outside the enclosures immediately after they were established (Juniper, unpublished data). After month, diatom densities were significantly different between enclosures (Kruskal-Wallis -way ANOVA, P<0.05). Cell Table 3 Species richness (number of taxa, S), Shannon- Wiener Information Index (W; bits per individual) and Evenness (f) of the diatom community in the experimental cages, Avon-Heathcote estuary, 5 May 98. A similarity index (range 0-.0) compares the community structure at different snail densities on a pair-wise basis. s IT r SIMI Excl-Cages Excl-Cage vs 2 x-cage 0.88 N-Cages Excl-Cage vs N-Cage x -Cage N-Cage vs 2 x-cage 0.92 densities were notably higher in the Excl-Cages (0 0 ± 822 valves per mm 2 ) than in the 2 x -Cage (6088 ± 599 valves per mm 2 ), with densities in the N- Cage (7905 ± 874 valves per mm 2 ) intermediate. When the ratios of diatom fragments to whole diatom cells in the foregut and faeces of Amphibola were assessed, relatively few broken diatom valves were found in the pre-gizzard foregut of the snail (0.52), compared to the ratio in the faeces (2.42). This clearly indicates that diatom valves are ground up in the passage through Amphibolous gut. Some diatoms in Amphibola faeces had escaped digestion and had intact chloroplasts. Preliminary culture experiments showed that some unidentified diatoms remained viable after passage through the snails' gut and egestion in the faeces. DISCUSSION The numerically dominant assemblage (defined as those species comprising more than % of the community; Fig. 5) is probably characteristic of the habitat sampled in the estuary. Sullivan (975) found that any given slide of diatoms from a Delaware salt marsh could be identified with its habitat of origin by the association of dominant species. He concluded that each of the habitats in the marsh produced its own distinct and easily recognisable edaphic diatom community over an entire yearly cycle. However, numerous studies (Mclntyre 973; Sullivan 975, 977, 978) have demonstrated marked differences in the association of dominants, the number of endemic species, number of taxa, diversity, and evenness between different habitats in an estuary. Consequently it is expected that other Avon-Heathcote estuary diatom assemblages will differ markedly from that described in this paper. The habitat examined here supports a number of

10 McClatchie et al. Mudflat diatom community structure 307 Table 4 Relative abundance of benthic diatom taxa from all experimental cages combined in the Avon-Heathcote estuary 5 May 98. Relative abundance of taxa is based on a total count of 2233 valves (563 identified). Taxon No. of relative valves abundance counted (%) Taxon Achnanthes conspicua var brevistriata Hust. A. hauckiana Grun A. hauckiana var. rostrata Schulz. 0 A. lanceolata Grun. 3 A. levanderi Hust. 5 A. wellsiae Reimer 20 Achnanthes sp. A. Achnanthes spp. indeterminate 2 Amphiprora spp. indeterminate 4 Amphora coffeaeformis (Agardh) Kutz. 76 A. perpusilla Grun. Amphora sp. A. Amphora sp. B. Amphora spp. indeterminate Cocconeis disculus (Schumann) Cleve C. placentula var. euglypta (Ehr.) Grun. C. scutellum Ehr. C. thumensis Mayer Cocconeis sp. A. Coscinodiscus spp. indeterminate Cyclotella caspia Grun. Cymbella microcephala Grun. Cymbella spp. indeterminate Diploneis interrupta (Kutz.) Cleve D. oblongella Naegeli Dimerogramma spp. indeterminate Fragillaria construens var. venter (Ehr.) Grun. Gomphonema littorale G. olivaceum var. minutissima Hust. Gomphonema spp. indeterminate Gyrosigma fasciola Ehr. G. peisonis Grun. Licmophora Sp. A Navicula confervacea var. peregrina Grun. N. diserta Hust. N. luzonensis Hust. N. mendotia Vanlandringham N. mutica var. cohnii (Hilse) Grun. N. radiosa Kutz. N. radiosa var. parva Wallace N. tenera Hust. N. vaucheriae Hust. Navicula sp. A. Navicula sp. B. Navicula spp. indeterminate Nitzchia apiculata (Greg.) Grun. N. fonticola Grun. N. frustulum (Kutz.) Grun. N. hantzchiana Rabh. N. inconspicua Grun. N. longissima (Brebisson) Grun. N. ovalis Arnott N. romana Grun. N. thermalis var. minor Hilse Nitzchia sp. (Triblionella section) Nitzchia sp. A. Nitzchia spp. indeterminate Opephora martyi Heribaud O. parva (Grun.) Krasske Plagiogramma leve (Greg.) Ralfs Plagiogramma appendiculatum Giffen Rhabdonema crassum Kutz. Rhaphoneis amphicews Ehr. Synedra fasciculata var. truncata (Grev.) Patrick Unidentifiable valves Cells in girdle view Total No. of relative valves abundance counted (%) diatom taxa comparable with the range found in, North American estuaries. Sullivan (975) found between 57 and 62 species in the edaphic communitees associated with grassy areas in a Delaware salt marsh, whereas a bare bank lacking macroscopic vegetation supported 43 species, and a salt pan only 30 species. In the present study, a total of 53 species was identified. Diversity calculated for the Avon-Heathcote habitat (H = ± 0.72 SD bits per individual) agrees well with Sullivan's (975) bare bank community (H == ± 0.239), and lies within the range calculated by him for 5 habitats in a Delaware (USA) salt marsh (H 2.648^.688). It may be argued that describing the structure of the edaphic community within the enclosures may be artifactual, in that the enclosures themselves modify the habitat. A number of authors (Virnstein 978; Hulberg & Oliver 980) pointed out that most investigations involving caging experiments have neglected to consider sedimentary habitat modifications created by cages. Enclosures can potentially interrupt water flow and thus modify the hydrographic environment (Hulberg & Oliver 980). Water flow may also be modified by plants and animals which foul the cages. Depending on the degree of water movement, cages may induce scouring or alter the depositional environment leading to the accumulation of finer sediments within the enclo-

11 308 New Zealand Journal of Marine and Freshwater Research, 982, Vol. 6 While the above criticisms are valid for a number of studies, they are of lesser importance to the present one for the following reasons. Most of the problems have arisen with small-sized predator exclusion cages which are radically different from the enclosures used in this study. Our enclosures were large (4.0 m 2 ), compared with cages m 2 or less as used in other studies, and more importantly, they were not roofed over. In addition, the enclosures were kept cleared of any seaweed that accumulated. In a stud}' of the deposition and erosion of sediments caused by cages of different sizes and shapes on soft substrates, Hulberg & Oliver (980) found that small-cylinder cages trapped the greatest sediment deposit, an intermediate size collected less, and no measurable deposit accumulated in large-cylinder cages. These large cages were 3 m in diameter (7.069 m 2 ) with walls.0 m high. Our enclosures were large (4.0 m) with walls only 0.20 m high, and so would be expected to have little effect on the depositional environment, and also would not be tall enough to shade the substrate. Furthermore, Hulberg & Oliver (980) found that cage-related deposition increased with exposure interval. These authors sampled their large cylinder cages after 3-4 months, but were unable to detect any increase in the: silt-clay fraction in their largest cages. In our study, samples were taken after only month, giving less time for cage-related depositional effects to occur. We can therefore conclude that the absence of detectable fine sediment deposition in our enclosures was in fact reasonable, and could be expected from an extrapolation of Hulberg & Oliver's (980) results. Although Pace et al. (979) observed that snails tended to aggregate at the; walls of their enclosures, we saw no evidence that this occurred in our enclosures. Our information suggests that the size and form of our enclosures minimized enclosurerelated artifacts. Amphibola passes large volumes of sediment through its gut, up to 24 times its own body weight per day (Briggs 972). Its gizzard is large and muscular, although toothless (Jolly 97). Contractions of this organ would produce a highly abrasive action between sediment particles and diatoms, facilitating their fragmentation. As our comparison of the ratio of broken to v/hole diatom valves in the faeces and crop has shown, Amphibola can effectively break up diatoms, That some cells remain viable after gut passage could be significant to the diatom community structure if they pass back into the sediments while species more vulnerable to fragmentation are removed. However, more controlled studies with finer spatial scale are necessary to resolve the impact of Amphibola on the mudflat diatom community. 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