(Yendo) Fensholt on the Seagrass Zostera marina L

The Effect of the Invasive Alga Sargassum muticum (Yendo) Fensholt on the Seagrass Zostera marina L. and its Associated Fauna By

James Richard Tweedley

Thesis submitted to the University of Plymouth in partial fulfilment of the requirements for the degree of MRes Marine Biology

University of Plymouth Faculty of Science

September 2006

Copyright Statement: This copy of the thesis has been supplied on the condition that anyone who consults it is understood to recognise that its copyright rests with the author and that no quotation from this thesis and no information derived from it may be published without the author’s prior written consent.

2

The Effect of the Invasive Alga Sargassum muticum (Yendo) Fensholt on the Seagrass Zostera marina L. and its Associated Fauna

By

James R. Tweedley

Abstract: The first reports of attached Sargassum muticum occurring in British waters coincided with a warning that this invasive alga may replace the seagrass Zostera marina. While many subsequent studies have investigated the biology of the alga and the effects of the introduction of the indigenous algal and the associated faunal community, few have responded to the original warning. Presented here are three studies into the interactions between the two species and any effects on the associated fauna of Z. marina. I show that contrary to previous thoughts, S. muticum can inhabitat soft substratum and successfully attach and grow within a Z. marina bed, although the size of the plant is reduced. Moreover, it may be the presence of the Zostera itself which enhances S. muticum settlement. Growth and photosynthetic responses to inter and intra-specific competition in each species were also performed. Although the growth measures were shown to be unaffected by the presence of the other species, a reduction in photosynthetic efficiency was demonstrated in both species when in competition with the other. The fauna of a range of vegetation types containing both Z. marina and S. muticum were sampled using both FBA and seine nets over a four month period, to assess the effect of S. muticum on faunal assemblage structure within two Zostera beds in South-West England. S. muticum stands and mixed areas of Z. marina and S. muticum were found to contain a richer, more abundant, and faunally distinct community compared to Zostera beds. The implications of the S. muticum invasion on Z. marina and the associated fauna are discussed with relation to the original warning that S. muticum may replace Z. marina beds.

3

Contents: Chapter:

Page:

1. General Introduction:

9

2. Site Description:

13

3. The Occurrence of ‘Attached’ Sargassum muticum Plants Within a Zostera marina bed in South-West England: 15 3.1. Introduction: 15 3.2. Materials & Methods: 16 3.2.1 Sampling Procedure 3.2.2 Statistical Analysis

16 16

3.3 Results:

Substrates

17

3.3.1 Substrate Preference of S. muticum in Both Sargassum and Zostera Beds 17 3.3.2 Length Differences Between S. muticum Plants Attached to Hard or Soft 18

3.4 Discussion:

19

3.4.1 The Attachment Mechanism of S. muticum in Soft Substrates 3.4.2 The Effect of Attachment Substrate on Plant Length 3.4.3 Implications of S. muticum Growing Within Z. marina Beds

19 21 22

4. The Effects of Inter and Intra-specific Competition between Zostera marina and Sargassum muticum on Growth Measures and Photosynthetic Efficiency Over a Range of Salinities: 23 4.1 Introduction: 23 4.2 Materials & Methods: 26 4.2.1 Plant Material 4.2.2 Growth Experiment 4.2.3 Statistical Analysis

4.3

26 27 28

Results:

29

4.3.1 4.3.2 4.3.3 4.3.4

29 29 30 32

Growth and Photosynthetic Measurements at a Salinity of 15 Growth and Photosynthetic Measurements at a Salinity of 25 Growth and Photosynthetic Measurements at a Salinity of 35 Growth and Photosynthetic Measurements Between Salinities

4.4 Discussion:

36

4.4.1 The Effect of Inter and Intra-specific Competition Between Z. marina and S. muticum on a Range of Growth Measures: 36 4.4.2 The Effect of Inter and Intra-specific Competition Between Z. marina and S. muticum on Photosynthetic Efficiency: 38

4

Contents cont: Chapter:

Page:

5. The Effect of Sargassum muticum on the Mobile Epifauna and Epibenthos of Zostera marina Beds in South West England: 41 5.1 Introduction: 41 5.2 Materials & Method: 44 5.2.1 Sampling Methodology 5.2.2 FBA Net Sampling Protocol 5.2.3 Seine Net Sampling Protocol 5.2.4 Sorting and Species Identification 5.2.5 Statistical Analysis

5.3 Results:

44 44 45 45 45

47

5.3.1 Abundance and Diversity Measures of FBA Net Data 47 5.3.1.1 Number of Individuals (n) 47 5.3.1.2 Number of Species (S) 49 5.3.1.3 Diversity Indices 50 5.3.2 Mobile Epifauna Host-Vegetation Specificity 52 5.3.3 FBA Net Faunal Community Analysis Comparisons Between March and May 2006: 53 5.3.4 FBA Net Faunal Community Analysis March 2006 54 5.3.5 Seine Net Faunal Community Analysis May 2006 58 5.3.6 Abundance and Diversity Measures of Seine Net Data 61 5.3.6.1 Number of Individuals (n) 61 5.3.6.2 Number of Species (S) 64 5.3.6.3 Diversity Indices 64 5.3.7 Mobile Epibenthos Host-Vegetation Specificity 67 5.3.8 Seine Net Faunal Community Analysis Comparisons Between April and June 2006: 68 5.3.9 Seine Net Faunal Community Analysis April 2006 69 5.3.10 Seine Net Faunal Community Analysis June 2006 70

5.4 Discussion:

74

5.4.1 The Influence of Vegetation and S. muticum in terms of Mobile Epifauna and Epibenthic Abundance, Richness and Diversity 74 5.4.2 The Influence of S. muticum on Faunal Community Assemblages within Z. marina Beds 77 5.4.3 Consequences of the Introduction of S. muticum on the Fauna Inhabitating the Zostera Beds at Salcombe 79

6. General Conclusion:

81

7. Appendices:

85

7.1 FBA Net: Average Species Abundance for each Mobile Epifaunal Species in the Vegetation Types 85 7.2 Seine Net: Average Species Abundance for each Mobile Epibenthic Species in the Vegetation Types 88

8. References:

91

5

Figure Listing: Figure:

Page:

Chapter 2: Site Description 2.1: Study Site Location Map

14

Chapter 3: The Occurrence of „Attached‟ Sargassum muticum Plants Within a Zostera marina bed in South-West England 3.1: Substrate Preference of S. muticum Plants 17 3.2: Mean Length of S. muticum Plants Occupying Different Substrate and Vegetation Types 18

Chapter 4: The Effects of Inter and Intra-specific Competition between Zostera marina and Sargassum muticum on Growth Measures and Photosynthetic Efficiency Over a Range of Salinities 4.1:Photosynthetic efficiency of Z. marina and S. muticum in different plant mixes and salinities 31 4.2(a): Z. marina: Leaf Length Against Salinity and Plant Mix Treatment 33 4.2(b): Z. marina: Leaf Width Against Salinity and Plant Mix Treatment 33 4.2(c): Z. marina: Wet Biomass Against Salinity and Plant Mix Treatment 33 4.2(d): Z. marina: Wet Detritus Against Salinity and Plant Mix Treatment 33 4.3(a): S. muticum: No of Fronds Against Salinity and Plant Mix Treatment 34 4.3(b): S. muticum: No of Branches Against Salinity and Plant Mix Treatment 34 4.3(c): S. muticum: Total Length Against Salinity and Plant Mix Treatment 34 4.3(d): S. muticum: Wet Biomass Against Salinity and Plant Mix Treatment 34 4.4: S. muticum: Photosynthetic Efficiency Against Salinity and Plant Mix Treatment 35

Chapter 5: The Effect of Sargassum muticum on the Mobile Epifauna and Epibenthos of Zostera marina Beds in South-West England 5.1: FBA Net: Number of Individuals Caught in Different Vegetation Types 5.2(a): FBA Net: Number of Species Caught in Different Vegetation Types 5.2(b): FBA Net: Margalef‟s Species Richness in Different Vegetation Types 5.2(c): FBA Net: Pielou‟s Species Evenness in Different Vegetation Types 5.2(d): FBA Net: Shannon Diversity in Different Vegetation Types 5.3: FBA Net: MDS Ordination for Comparing March and May 2006 5.4: FBA Net: MDS ordination for March 2006 5.5: FBA Net: MDS ordination for May 2006 5.6: Seine Net: Number of Individuals Caught in Different Vegetation Types 5.7(a): Seine Net: Number of Species Caught in Different Vegetation Types 5.7(b): Seine Net: Margalef‟s Species Richness in Different Vegetation Types 5.7(c): Seine Net: Pielou‟s Species Evenness in Different Vegetation Types 5.7(d): Seine Net: Shannon Diversity in Different Vegetation Types 5.8: Seine Net: MDS Ordination for Comparing April and June 2006 5.9: Seine Net: MDS ordination for April2006 5.10: Seine Net: MDS ordination for June 2006

6

49 51 51 51 51 53 54 58 62 66 66 66 66 68 69 70

Table Listing: Table:

Page:

Chapter 1: General Introduction 1.1: Functions and values of seagrasses from an ecosystem perspective

11

Chapter 4: The Effects of Inter and Intra-specific Competition between Zostera marina and Sargassum muticum on Growth Measures and Photosynthetic Efficiency Over a Range of Salinities 4.1: GLM results for growth and photosynthetic efficiency measures against treatment and salinity 32

Chapter 5: The Effect of Sargassum muticum on the Mobile Epifauna and Epibenthos of Zostera marina Beds in South-West England 5.1: FBA Net: Total Number of Species and Individuals in Different Vegetation Types 5.2 MANOVA of FBA Net Abundance and Diversity Measures 5.3 Number of Unique and Shared Species Between Vegetation Comparisons 5.4: FBA Net: Species Similarity Within Vegetation Types for March 2006 5.5: FBA Net Species Dissimilarity Between Vegetation Types for March 2006 5.6: FBA Net: Species Similarity Within Vegetation Types for May 2006 5.7: FBA Net Species Dissimilarity Between Vegetation Types for May 2006 5.8: Seine Net: Total Number of Species and Individuals in Different Vegetation Types 5.9 MANOVA of Seine Net Abundance and Diversity Measures 5.10 Number of Unique and Shared Species Between Vegetation Comparisons 5.11: Seine Net: Species Similarity Within Vegetation Types for June 2006 5.12: Seine Net Species Dissimilarity Between Vegetation Types for June 2006

48 48 52 56 57 59 60 63 63 67. 72 73

Chapter 7: Appendices 7.1 FBA Net: Average Species Abundance for each Mobile Epifaunal Species in the Types 7.2 Seine Net: Average Species Abundance for each Mobile Epibenthic Species in the Types

7

Vegetation 85 Vegetation 88

Acknowledgements: The author would like to thank Professor Martin Attrill and Dr Emma Jackson for their help and support during the planning and manuscript preparation and Richard Ticehurst, Penelopi Goumenaki, Charlotte Goswell, Sophie Mowles, Samantha Glanfield and Ann Torr for their help when conducting fieldwork; without whom this project would not have been completed.

8

Chapter 1: General Introduction:

Sargassum muticum (Yendo) Fensholt is a branched perennial brown alga originating from Japan and is one of around 400 species in the Sargassum genus (Phaeophyta, Fucales). In Japan S. muticum is an innocuous member of the algal community. However, since the 1950s, the alga has been far from innocuous, spreading across two continents outside of its natural range, a phenomenon only achieved to date by Codium fragile subspecies tomentosoides. This spreading has earned S. muticum a reputation as being both invasive and the 8th most hazardous alga in Europe (Nyberg and Wallentinus, 2005).

S. muticum was first reported outside of its natural range when the alga was found by Fensholt in British Columbia in 1955. Since then the alga extended its range on the Pacific coast of America, from Vancouver Island in the North (Davidson, 1999) to Mexico in the South; where it appears to have reached its geographical limit (Espinoza, 1990). Druehl (1973), after observing the alga in British Colombia warned that following the introduction of oyster spat (Crassostera gigas) into France that S. muticum would establish itself in Europe: that same year attached specimens were recorded in Europe for the first time on the Isle of Wight (Farnham et al. 1973). By 1990 the alga has spread to Scandinavia, Belgium, The Netherlands, and France (Critchley, et al.1990 and the references within). Later reports show that the alga has established itself in the Mediterranean, colonising areas in France (Rueness, 1989), Spain (Fernández et al., 1990), Portugal (Sánchez et al., 2005) and Italy (Curiel et al., 1998).

The capacity of S. muticum to spread and invade new areas seems to rely on: (i) a high reproductive potential (Norton, 1977b) coupled with both self-fertility and a significantly shorter fertility time than indigenous algae (Norton, 1976); (ii) a range of dispersal methods including, germlings, „stonewalking‟ (see Critchley, 1983a), while detached fronds remain viable for three months and become fertile (Norton, 1981); (iii) a fast growth rate of up to 4 cm a day (Davidson, 1999), which may be up to three times faster than indigenous algae (Norton, 1977) and allow the alga to blanket the shore overshadowing other species (Norton, 1976).

9

Various studies have investigated the effects of S. muticum on the indigenous community; however, in virtually all studies the community investigated has been algal. S. muticum has been shown to reduce light levels (Critchley et al. 1990b) and also reduce current flow, increase sedimentation and reduce nutrient levels (Britton-Simmons, 2004) in areas where it is present. Competition for light has been used to explain reductions in dominant native algae (Sάnchez et al. 2005) including the giant kelp Macrocystis pyrifera (Ambrose and Nelson, 1982) in areas where S. muticum has invaded. Other studies have also reported a significant change in the local algal community (Viejo, 1997; Stæhr, 2000), and in densities and richness of indigenous algal species (Britton-Simmons, 2004).

Conversely, studies investigating the effect of S. muticum on epiphytes and fauna indicate that S. muticum can support epibonts (Withers et al. 1975) and that in areas of similarly complex algae S. muticum harbours a similar associated community (Viejo, 1997; Wernberg et al. 2004), but when compared to algae of a lesser complexity S. muticum harbours a richer and more abundant fauna (Buschbaum et al. 2006). Sargassum species have also been shown to harbour higher fish densities than both rocky and bare areas (Ornellas and Coutinho, 1998). However, Druehl (1973) warned that the alga „has become well established‟ in areas normally occupied by the seagrass Zostera marina, and that if Sargassum is replacing Zostera the results would be disastrous. Seagrasses are a group of marine angiosperms encompassing around 60 species of which Zostrera marina is one (Spalding et al. 2003); they perform a number of ecosystem functions which make them a valuable habitat (Table 1.1). Therefore, any threat resulting in the loss of seagrass habitat should be taken seriously. For example, Druehl (1973) was concerned about the loss of Z. marina as a nursery ground for marine species; studies have confirmed that seagrass ecosystems act as a nursery ground (see Heck et al. 2003) and that the reduction of seagrasses leads to a shift in fish assemblage with a loss of taxa at the family level (Pihl et al. 2006), which may have dramatic effects on fish population dynamics and ecosystem functioning.

10

Table 1.1: Functions and values of seagrasses from an ecosystem perspective. (Modified from Short et al. 2000 and Spalding et al. 2003). Ecosystem Function Primary production

Value Seagrasses are highly productive and are feed on by various herbivores (fish, turtles, marine mammals and birds).

Canopy structure

Seagrasses provide a complex 3-d environment providing a habitat and nursery ground for various species.

Epiphyte and epifaunal substratum

Seagrasses provide large surface areas for epiphytes and epifauna, supporting high secondary productivity.

Nutrient and contaminant filtration

Seagrasses help to settle and remove contaminants from the water column and sediments.

Sediment trapping

The canopy of seagrasses helps encourage settlement of sediments and prevent resuspension; while the roots and rhizome bind the sediment together.

Oxygen production

Oxygen released from photosynthesis helps to improve water quality and support faunal communities.

Organic matter production, accumulation and export

Detritus produced by the plants is exported supporting offshore productivity.

Nutrient regeneration and recycling

Seagrasses themselves hold nutrients and help nutrient recycling.

Reducing wave and current energy

By binding the sediment sediments and preventing the scouring of waves directly on the benthos seagrasses reduce the effects of wave and current energy, reduce erosion and increase sedimentation.

Carbon sequestration

As perennial structures seagrasses store carbon for relatively long periods.

Comparing the distribution map of S. muticum to the distribution of seagrasses some 7,840 km2 of seagrass beds may be under threat, which equates to between 1.5 and 5% of world seagrass, depending on the estimate of the worldwide seagrass distribution (Green and Short, 2003). Therefore, it is surprising that few people have investigated interaction between these two species. den Hartog (1997) asked the question, „Is S. muticum a threat to seagrass beds?‟; he agreed with previous studies (Norton, 1973), in stating that the two species may not be in direct competition due to the substrate preference of either species: Z. marina for soft substrata, while S. muticum prefers hard substrate. However, in areas of mixed substratum and where Z. marina has been in decline, S. muticum may colonise thus interfering with the regeneration of the bed (den Hartog, 1997).

11

In the Salcombe-Kingsbridge estuary (Devon, South West England) that the two species occur in both in mono-specific and mixed patches, thus providing the ideal opportunity to investigate the effect of this invasive alga on the eelgrass itself and the associated fauna. This thesis aims to investigate the interaction between these two species on three levels. (i)

Spatial separation; investigate the substrate preference hypothesis, does S. muticum only occur on hard substrate or can it grow within a Zostera bed and if so how does it attach?

(ii)

Inter-specific vs. intra-specific competition; does the presence of either species effect the growth or photosynthetic rates of the other?

(iii)

The effect on the biodiversity and richness of the associated fauna; is the faunal community affected by the addition of S. muticum?

The following three chapters each describe a study designed to answer one of the questions above, while the next summarises each of the studies and discusses them in context with the overall question „what effect does the invasive alga S. muticum have on the seagrass Zostera marina and the associated fauna?‟.

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Chapter 2: Study Site:

The seagrass meadows where the interactions between S. muticum and Z. marina were studied are located within the Salcombe-Kingsbridge estuary in Devon (South West England) (Fig 2.1). The estuary is a complex dendritic ria-formed estuary with an unusual set of physical and geographical conditions (Anon, 2005a). The topography of the estuary makes it fairly sheltered with minimal freshwater input, the estuary being almost fully marine with a salinity at high tide of 34-35‰ (Snowdon, unpublished data). The shores are gently sloping with a maximum depth of 12.5m, with water temperature varying between 10 – 17 °C during the course of the experiments. Tides are semi-diurnal with a mean tidal range within the estuary of 4.6m and 2.0m at spring and neap tides respectively (Anon, 2005b). Two sites were used in the experiments: the Harbour (50°235‟N; 003°76‟W) and Castle Bay (50°231‟N; 003°77‟W) (Fig.2.1), both of which are situated on the West bank of the Salcombe-Kingsbridge estuary and separated by Salcombe castle and an extensive kelp (Laminaria digitata) stand.

Total seagrass extent at Salcombe is estimated at 6.3 ha, with 2.5 ha occurring at Harbour and 2 ha at Castle Bay (Jackson, unpublished data). The Z. marina meadows on Castle Bay and the Harbour occur in the upperlittoral in a predominantly sandy/silty sediment; where the average gain size is 513 and 283μm respectively (Goumenaki, unpublished data). S. muticum is present occurring in low littoral rock pools in Castle Bay and in mooring scars and rocky patches in the sublittoral of both beaches. Z. marina density is constant between the two bays with an average shoot density of 240 shoots m2 (Goumenaki, unpublished data).

13

Salcombe Harbour To Kingsbridge

Harbour

Salcombe Castle

Castle Bay

To the English Channel

Fig 2.1: The location of the Salcombe-Kingsbridge estuary with England and Wales and a seagrass map of the seaward end of the estuary, showing the location of Salcombe town and harbour and the two sample sites Castle Bay (50°231‟N; 003°77‟W) and the Harbour (50°235‟N; 003°76‟W). Note the presence of both seagrass and drift algae at both sites. Source: Modified from Emma Jackson. 14

Chapter 3: The Occurrence of „Attached‟ Sargassum muticum Plants Within a Zostera marina bed in South-West England 3.1 Introduction: In 1973 concerns were first raised about the possible ecological effects of S. muticum when Druehl (1973) expressed fears that S. muticum may displace seagrass beds. His evidence was based on observations that in British Columbia the alga had become „well established in sheltered low intertidal and shallow subtidal waters‟, a position usually occupied by Z. marina, which provides an important nursery habitat for juvenile fish (Jackson et al. 2001). Z. marina has been shown to harbour a larger number of individuals and species than surrounding areas (Polte et al. 2005; Polte and Asmus, 2005; Pihl et al. 2006) North (1973) disputed the concerns made by Druehl (1973) after examining S. muticum patches in California, where he claimed that, „the alga requires a solid substrate for attachment‟. With Z. marina occurring in soft sediment he concluded that in California that „the two species coexist without apparent competition‟, thus allaying fears that S. muticum may replace Z. marina. Subsequent studies on S. muticum also reported that the alga requires a hard substrate, e.g. bed-rock, boulders, stones or solid man-made structures (Fletcher and Fletcher, 1975, Norton, 1977), with stones (0.05) and Bartlett's tests (ά >0.05)

respectively. In all cases the assumptions of normality and

homogeneity could be met, thus allowing the use of parametric tests. All tests were performed using Minitab version 13.20 (Minitab Inc).

16

3.3 Results:

3.3.1 Substrate Preference of S. muticum in Both Sargassum and Zostera Beds: In total, 110 S. muticum plants were sampled, of which 77 occurred outside the Zostera bed and 33 inside. Of the 77 that occurred in the Sargassum bed, 75% were attached to hard substrate; conversely, inside the Zostera bed, 91% of the plants were attached to soft substrate. A Chi-Square test proved significant (χ2= 41.023, P= 0.1): indicating that the location of the plant with relation to the Zostera bed has no significant effect on the length of the S. muticum plant. A significant difference was observed when comparing the mean length of S. muticum plants in different substrates, with plants attached to hard substrate having a mean length of 43.47 ± 2.28 cm compared to 29.18 ± 2.91cm when attached to soft substrate (one-way ANOVA P= 0.05 for substrates). Conversely, when comparing different substratum types to length in similar vegetation, significant differences were found between substrate difference in Sargassum and Zostera beds (Student t-test, P= 10 cm in diameter). In Castle Bay stony/rocky patches exist in the sublittoral and are mainly colonised by S. muticum and L. digitata attached to the hard substrate, with no Z. marina found (Tweedley, pers. obs.). However, in the sandy low littoral areas, there are very few patches of hard substrate and the area is dominated by Z. marina beds, meaning that any S. muticum plants within the Zostera bed must be buried within the soft substrate as there is no hard substrata to attach (average grain size is 159 μm (Goumenaki, unpublished data)). However, significantly fewer S. muticum plants were recorded in sandy areas outside the Zostera bed than attached to hard substrate, suggesting that without the Zostera bed S. muticum plants are less likely to successfully attach to or become buried within the soft substrata.

Significant differences were found between the length of plants growing in the different substrates, with plants attached to hard substrate being longer. One possible reason for this may be that larger plants become buoyant and „break free‟ from their attachment from within rhizome mat or sediment of the Zostera bed. S. muticum has many small air bladders (2-6 mm) which are present on the axil of leaf lamina on all laterals (Davidson, 1999) and provide buoyancy. As mentioned before, the plants provide enough buoyancy that at times they are able to „lift‟ the hard substrate they are attached to a phenomenon know as „stonewalking‟ (see Crichley, 1983d). Therefore, it is conceivable that larger, more buoyant plants may become detached during turbulent hydrographic conditions, thus leaving only the smaller less buoyant plants buried in the soft sediment, an observation noted by Critchley (1983c). No significant difference was found between the lengths of S. muticum plants attached to similar substrate in different vegetation types; indicating that that is the substrate that 21

dominates the growth of the plant and not the surrounding conditions. This is consistent with the work of North (1973); Norton (1977) and den Hartog (1997), who stated that the plant can co-exist with Z. marina without any apparent direct competition. However, this does not mean there is not any competition, and these results only show that at Castle Bay the mean length of S. muticum plants remained consistent regardless of the surrounding vegetation. As S. muticum is an opportunistic species and rapidly colonises any free space (Critchley, 1983a), it is therefore, in indirect competition with Z. marina. After S. muticum settles in an area previously colonised by Z. marina it interferes with the regeneration of the bed and currently no germlings of Z. marina have been found in S. muticum stands (den Hartog, 1997). This is similar to reports of competition between S. muticum and the giant kelp Macrocystis pyrifera, where once S. muticum has settled it prevents re-establishment of the indigenous M. pyrifera (Ambrose and Nelson, 1982).

3.4.3 Implications of S. muticum Growing Within Z. marina Beds: In conclusion, this study demonstrates that the presence of Zostera beds may enhance the settlement of S. muticum plants allowing plant to settle in otherwise unsuitable substratum. Rather alarmingly, den Hartog (1997), forecasted that, „in time one may expect that S. muticum will replace the eelgrass (Z. marina) bed‟. However, his prediction was based on a decline in the Zostera beds due to disease, environmental, and anthropogenic effects rather than just interactions with S. muticum. If the presence of S. muticum attached within Zostera beds shown in the study is found to occur in other locations it may be possible that the prediction made by den Hartog (1997) may occur more rapidly than previously thought. Further research is needed into this area before any conclusions can be drawn, with more sites sampled to see if S. muticum’s ability to settle in soft sediment is prevalent just within the Salcombe-Kingsbridge estuary due to any topographic, hydrographic, and climatic conditions or does this phenomenon occur at other locations.

22

Chapter 4: The Effects of Inter and Intra-specific Competition Between Zostera marina and Sargassum muticum on Growth Measures and Photosynthetic Efficiency Over a Range of Salinities.

4.1 Introduction: Zostera marina is the dominating seagrass in the Northern Hemisphere, where it provides an important nursery habitat for juvenile fish (Jackson et al. 2001), often harbouring larger number of individuals and species than surrounding areas (Polte et al. 2005; Polte and Asmus, 2005; Pihl et al. 2006). However, since the introduction of the invasive alga S. muticum into Europe in 1973 (Farnham et al. 1973) concerns have been raised about the possible displacement of Z. marina beds with S. muticum (Druehl, 1973). These fears were allayed later when North (1973) stated that the two species require different substrates, and that in California they, „coexist without apparent competition‟. However, recent evidence now suggests that, S. muticum can not only replace Zostera beds when the beds have declined, but that re-colonisation is unlikely (den Hartog, 1997). While the majority of studies since the introduction of S. muticum have either investigated the biology and ecology of the alga, or the possible effect of S. muticum on local algal flora and associated fauna, the effect of S. muticum on Z. marina itself remains relatively unstudied.

Conversely, competition between seagrasses and rhizophytic macroalgae are relatively well researched, especially interactions involving Caulerpa spp, (Ceccherelli et al. 2000; Davis and Fourqurean, 2001; Taplin et al. 2005), with a number of possible mechanisms of competition between algae and seagrasses proposed in the literature including: competition for light, nutrients, and space (Ceccherelli and Cinelli, 1997; Davis and Fourqurean, 2001; Stafford and Bell, 2006). The presence of Caulerpa has been implicated in seagrass declines (de villele and Verlaque, 1995), suggesting that rhizophytic macroalgae maybe capable of out-competing seagrass species. S. muticum however, is not a rhizophytic macroalgae and virtually no research has been carried out investigating competition between

non-rhizophytic

macroalgae

and

seagrasses,

probably

because

non-

rhizophyticmacroalgae were thought only to occur on hard substrate and therefore would not interfere with seagrass bed dynamics. 23

However, work has been carried out of competition between non-rhizophytic macroalgae especially after the introduction of S. muticum. The presence of S. muticum within native Danish algal communities was found to negatively affect algal community structure, with decreases in percentage cover of species belonging to the genera, Laminaria, Fucus, and Codium (Stæhr et al. 2000). S. muticum has also been reported to be able to replace the giant kelp (Macrocystis pyrifera), with shading suggested as the possible mechanism for the inhibition of M. pyrifera recruitment (Ambrose and Nelson, 1982). Critchley et al. (1990) reported that as few a eight S. muticum plants m2 could give 100% surface cover and reduce light attenuation in the top 0.1 m of the canopy by 97% of the surface photosynthetically active radiation (PAR), causing a 9.2°C temperature decrease at a depth of 0.5 m. This combination resulted in the decreased size and frequency of indigenous algae (Critchley et al. 1990). In the San Juan Islands (USA) competition with S. muticum reduced the abundance of native canopy algae by approximately 75% and native understory algae by about 50%, while also reducing the species richness of canopy algae (Britton-Simmons, 2004). Britton-Simmons (2004) suggested the negative effects of S. muticum on the native algal community were due to shading rather than changes in water flow, sedimentation, or nutrient availability

As well as competition with macroalgae, salinity is considered to have an effect on seagrass growth and survival. Z. marina is reported to occur in areas with a salinity of 5-35 (den Hartog, 1970). However, Z. marina, like many other seagrasses, has optimum conditions of full strength seawater (Biebl and McRoy, 1971). Although Z. marina can survive in suboptimal conditions (hypo and hyper saline), various studies have shown a reduction in: photosynthetic rates (Biebl and McRoy, 1971, Hellblom and Björk, 1999); productivity (Pinnerup, 1980); growth (Kamermans et al. 1999); metabolism (van Katwijk et al, 1999); reproduction (Ramage and Schiel, 1998); and survival (Vermaat et al. 2000) in various Zostera species occurring in sub-optimal salinities. S. muticum is also euryhaline, tolerating a salinity range of 5-35 but with an optimum ≈35 (Norton, 1977b). Critchley et al. (1990) warned that the alga is able to grow and reproduce at reduced salinities, making it a contender for estuarine invasions. With both Z. marina and S. muticum being euryhaline, competition may occur in a range of salinities depending on location. For example, both species are know to occur at oceanic salinities (≈35). In the 24

Solent (Farnham et al. 1973), and in reduced salinity environments like the Baltic, (Nielsen et al. 1995) where salinities are between 2-12 (Hellblom and Björk, 1999). The effects of competition between S. muticum and Z. marina are virtually unstudied in full strength seawater, with no reports comparing this in different salinities, whereas studies on competition between macroalgae and seagrasses have cited competition for nutrients or shading (Ceccherelli and Cinelli, 1997;Coffaro and Bocci, 1997; Davis and Fourqurean, 2001; Taplin et al. 2005) as possible competition mechanisms. Therefore, the objective of the present work to investigate the effects of inter and intra-specific competition between Z. marina and S. muticum on the growth rates and photosynthetic efficiency (PE) of both species over a range of salinities (15, 25, and 35) in a laboratory controlled experiment without competition for nutrients or light.

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4.2 Materials and Methods: 4.2.1 Plant Material: Intact vegetative Z. marina and S. muticum plants were collected from a shallow seagrass bed in the Kingsbury estuary, Devon, UK, (50°231‟N; 003°772‟W) in March 2006. The specimens were transported in seawater to a controlled temperature (CT) room at the University of Plymouth. The conditions in the room were kept constant, with a temperature of 15°C (the equivalent of the surface seawater temperature at the time of the experiment), which is within the temperature ranges of both species; S. muticum has a tolerance range of between 5-30°C (Norton, 1977b), as does Z. marina (den Hartog, 1970). The tanks were well aerated and illuminated under natural spectrum halogen lighting with a photoperiod of 12: 12 h light: dark.

Z. marina shoots were separated from their neighbouring shoots by cutting the rhizome, placed in 10 L tanks of filtered seawater with S. muticum plants also in separate 10L tanks, and allowed to recover for a two weeks. The two species were cultured separately, with 6 plants per tank. After the recovery period plants were transferred to their experimental salinity (15, 25, or 35) and left to acclimatise for two weeks, because acclimation to salinity variations allows a greater tolerance to osmotic stress than instantaneous transfer (Ralph, 1998). The experimental water was made-up to the desired salinity by adding a nitrate and phosphate free artificial salt mix (Instant Ocean®, Aquarium Systems, Ohio, USA) to deionised freshwater and tested using a hand-held optical refractometer (Bellingham and Stanley, Kent, UK). The salinity of the experimental water was controlled and maintained (±1) and changed every 14 days.

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4.2.2 Growth Experiment: The experiment had two variables: water salinity and plant mix. Three different salinities were used: 15, 25 and 35. Each salinity had three different plant mixes (hereby referred to as treatments): tanks with two Z. marina plants; two S. muticum plants; and tanks with one of each plant. Each treatment was replicated three times, giving a total of 27 tanks. A number of measures were used in order to assess the growth rate of both Z. marina and S. muticum. For Z. marina these included: dimensions of the longest leaf (length and width), wet weight of the plant biomass and detritus, and PE. The plastochrone interval method (Short and Duarte, 2001; Gaeckle and Short, 2002) was also used. However, it proved to be unsuccessful, as during the experiment some of the Z. marina plants shed the leaf with the syringe mark; leaving an insufficient number of plants for reliable statistical analysis. The number of fronds (laterals) and branches of S. muticum plants were also recorded along with the total plant length (including all laterals and branches), the wet weight of the plant biomass, and PE.

For the Z. marina specimens leaf length and width measurements were taken using a tape measure and calliper respectively; the same instruments were used to measure the length of each lateral and branch on the S. muticum specimens. The wet weight of each specimen was recorded by removing the plant from the water, drying the excess water, and weighing using an electronic balance; in the case of Z. marina any detritus was also collected and weighed. For S. muticum, total length was calculated by adding the length of all laterals and branches together. PE was measured using a Plant Efficiency Analyser (PAE) (Hanstech Instruments, Norfolk, UK).

While the growth measures indicated how the growth rates were affected by any possible interaction between Z. marina and S, muticum, recording the PE through-out the experiment aimed to determine any possible stress response in chlorophyll a levels. The PAE was used to measure the variable/maximum fluorescence ratio (Fv/Fm), which is calculated as: Fv/Fm = (Fm – Fo)/Fm. Where Fo is the minimum florescence, Fm the maximum, and Fv the maximum fluorescence yield of a dark adaptated sample. The Fv/Fm is the quantum efficiency of photosystem II (PSII) in dark adaptated samples and as been used as an indicator of stress (Ralph and 27

Burchett, 1995; Kamermans et al. 1999). Prior to analysis with the PAE two precursor tests were performed on each species: the first of which indicated how much of the 3000 μmol2 -1

s light intensity produced by the PAE the plants could adsorb before an over-scaling error

occurred; the second test indicated the dark-adaptation time which provided minimal variability between samples in the shortest time. Parameters of 100% light intensity for 5 seconds, with a dark-adaptation time of 15 minutes were selected for both species.

Prior to conducting the experiment, all plants were cleaned of epiphytes, and an initial/ baseline measurement for each of the above measures was recorded. The experiment ran for 6 weeks, with repetitions of the measuring process occurring on days 7, 21, and 42 into experimental culture with day 42 being the final measurement.

4.2.3 Statistical Analysis: Growth measures and photosynthetic efficiency rates were evaluated within each salinity by comparing plants of both species against the plant mix treatments using a one-way ANOVA. General linear models were used to compare the growth and photosynthetic measures between salinities and treatments. Preliminary data analysis through AndersonDarling (ά > 0.05) and Bartlett's tests (ά > 0.05) indicated that the assumptions of normality and homoscedasticity were not violated, thus allowing the use of parametric tests. All analyses were performed on Minitab version 13.20 (Minitab Inc).

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4.3 Results: 4.3.1 Growth and Photosynthetic Measurements at a Salinity of 15: Comparisons of the growth and photosynthetic responses of each species in the 15 vs. plant mix treatment (either monospecific (Z. marina or S. muticum) or mixed) showed no significant differences between any of the species at the start of the experiment (one-way ANOVA, P= >0.05 in all cases). No significant difference were recorded until six weeks into culture when the PE in S. muticum varied between plant mix treatments (one-way ANOVA, P= 0.05 in all cases). By the end of the experiment no significant differences were observed between plant mix treatments for any of the Z. marina (Fig 4.2) or S. muticum (Fig 4.3) growth measures or PE (Fig 4.1) (one-way ANOVA, P= >0.05 in all cases).

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4.3.3 Growth and Photosynthetic Measurements at a Salinity of 35: As with other salinity treatments, all of the growth and photosynthetic measures were similar across treatments prior to the start of the experiment (one-way ANOVA, P= >0.05 in all cases). No significant results were observed until week 6; where PE differed between treatments of both Z. marina and S. muticum (one-way ANOVA, P= 0.5 >0.5 0.5 > 0.5 > 0.5 >0.5 No data

S. muticum Growth and PE Measures No of Fronds No of Branches Total Length Plant Biomass Photosynthetic Efficiency

Treatment >0.05 >0.05 >0.05 >0.05 0.5 0.5