Mesograzers of Giant Kelp (Macrocystic Pyrifera) at Hopkins Marine Station, Pacific Grove, CA: Community Structure and Seasonal Patterns

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Mesograzers of Giant Kelp (Macrocystic pyrifera) at Hopkins Marine Station, Pacific Grove, CA: community structure and seasonal patterns

Amy Kirkham

May 2012

Acknowledgements

I am extremely grateful to all who made the completion of this undergraduate thesis possible. Many thanks to the generosity of the Vice Provost of Undergraduate Education and the Margaret Davis Collison Scholarship, which funded this project, and all the wonderful people who helped out along the way. I am grateful to Fiorenza Micheli, my research sponsor, for all her support throughout this process. Thank you so much. It has been such a pleasure to be part of the FioLab, and I have learned so much in working with you. I cannot say thank you enough to Sarah Lee and Steve Litvin for all they have done as my research mentors. Both have been unbelievably helpful in putting together this project at every step. Thank you for all your time and energy over email, Skype, and in person, and for getting me hooked on mesograzers in the first place. I would like to thank Peter Vitousek for all of his guidance as both my program advisor and second reader. Thanks also to Jim Watanabe for his help in drafting this thesis, as well as for showing me what a kelp forest and its invertebrates are up-close—I’m excited to stay in touch with the amphipods, isopods, and other beasties I met through this project in future underwater visits. Thanks are due to all members of the FioLab, human and canine, who kept the bench work for this project from ever getting too dull, and to the Hopkins faculty and staff that have made my undergraduate experience more than I could have hoped for. Thanks also to Gretchen Stumhofer for her help with quality control, Sarah Kaewert for her help with writing, my team and family for always being inspiring and supportive, and my parents especially for letting me leave the East Coast for school on the Pacific. It’s been worth it, I promise.

Table of contents

Abstract 1

Introduction 1 Kelp forest environment 1 Mesograzer community 2 Seasonal upwelling dynamics 2 The Hopkins Marine Life Observatory 3

Methods 4 Study site 4 Sampling method 4 Sample processing 5 Data analysis 5

Results 7 Community composition and monthly change 7 Patterns in abundance by size class 8 Community composition and upwelling 8

Discussion 9 Community composition and monthly change 9 Patterns in abundance by size class 10 Community composition and upwelling 11 1. Overall effects 11 2. Copepod abundance 11 3. Kelp density effects 12 4. Abundance patterns of larger grazers 13

Conclusions 15 Literature cited 17 Appendix 21 TABLES Table 1. Month-to-month comparison SIMPER results 21 Table 2. Monthly Pacific Fisheries Environmental Laboratory (PFEL) upwelling indices 21 Table 3. Average abundance of taxa in low and high upwelling months 22 Table 4. List of taxa observed in samples 23

FIGURES Figure 1. Total individuals observed in all samples by broad taxonomic group 24

Figure 2a. Monthly abundance of the most common taxa 24 Figure 2b. Monthly abundance of common taxa excluding copepods 25 Figure 3a. Monthly abundances of individuals less than 2mm in length 26 Figure 3b. Monthly abundances of individuals between 2 and 6mm in length 26 Figure 3c. Monthly abundances of individuals between 6 and 11mm in length 27 Figure 3d. Monthly abundances of individuals between 11 and 20mm in length 27 Figure 3d. Monthly abundances of individuals greater than 20mm in length 28 Figure 4. Monthly size class histograms of Idotea resecata 29 Figure 5. MDS plot of all samples labeled by the month of collection 30 Figure 6. MDS plot of all samples labeled by the relative upwelling conditions 30 Figure 7. MDS plot of monthly averaged abundances 31

ABSTRACT

This thesis describes the mesograzer community of the Hopkins Marine Station kelp bed canopy. The community is characterized in terms of both taxonomic and size class composition based on data from samples collected monthly from August 2010 to July 2011. Multivariate analysis revealed seasonal variation in community structure driven primarily by high numbers of copepods in the spring and summer. Larger-bodied taxa known to graze significantly on kelp exhibited higher abundances and sizes in winter months, especially the kelp isopod Idotea resecata. These changes in community structure were correlated with the intensity of seasonal upwelling. Changes in food quality associated with nutrients advected into the system by upwelling may have played a role in altering the relative abundances of different taxa. The increased density of large-bodied grazers in months following low upwelling may have been in part an artifact of their high mobility and a lower standing crop of kelp that coincided with these periods. Regardless of its ultimate cause, the co-occurrence of high mesograzer density and low nutrient availability synergistically elevates tissue degradation and loss by Macrocystis at this time of year with implications for both the kelp forest community and neighboring habitats.

INTRODUCTION

Kelp forest environment

The giant kelp (Macrocystic pyrifera) forests of California are extremely productive and diverse systems (Steneck et al. 2003). Through the provision of both fixed carbon and complex habitat, kelp forests support a large number of taxa within their nearshore subtidal environments. They also contribute to other communities’ food webs by serving as nurseries for many species and by exporting kelp detritus (Clark 1971, Steneck et al. 2003, Davenport and Anderson 2007). Macrocystis has been valued as an economic resource as well, for potash historically and more recently for algin, which is chemically useful in the production of many diverse materials (Foster and Schiel 1985). Kelp forests are also valued as tourist attractions and as protection against coastal erosion (Leet et al. 2001, Steneck et al. 2003). For these reasons, as well as the high

accessibility of their coastal locations, kelp forest ecosystems have been the subject of numerous and extensive ecological studies (North 1971a, Foster and Schiel 1985, Steneck et al. 2003, Graham 2004).

Mesograzer community

Kelp plants provide a large potential habitat for small motile organisms, with surface areas as much as fifteen times higher than that of the associated sea floor (Clendenning 1960, cited by North 1971) and five m2 per frond (Wing and Clendinning 1971). Among the array of organisms associated with Macrocystis is a collection of small mobile invertebrates, primarily crustaceans and mollusks, which live and feed on the surfaces of kelp blades (Coyer 1984, Foster and Schiel 1985). These organisms, known as mesograzers, can have large impacts on kelp community structure despite their relatively small biomass. Sala and Graham (2002) examined the interaction strengths between various mesograzers and kelp and found species capable of significantly impacting kelp recruitment. Isopods and amphipods, common mesograzer crustaceans, are capable of denuding kelp canopies when present in high densities (North 1971a, Foster and Schiel 1985, Graham 2002). Selective grazing may also impair kelp growth by lowering surface area to volume ratio and thus reducing nitrogen uptake (Bracken and Stachowicz 2007). Mesograzers may enhance grazing of other species as well, via indirect effects (Molis et al. 2010). Despite their significant role in kelp forest dynamics, mesograzers are less well studied than larger herbivores such as sea urchins because they are inconspicuous and difficult to sample (Duffy and Hay 2000, Davenport and Anderson 2003). Few long-term monitoring projects have examined Macrocystis canopy mesograzer communities, and those that have were based on southern Californian study sites (White and Clendinning 1971, Coyer 1979). Kelp forests north and south of Point Conception experience significant differences in environmental conditions (Graham et al. 2004) and may therefore host dissimilar canopy communities.

Seasonal dynamics and upwelling

The dynamics of northern California kelp forests are affected by annual cycles of coastal upwelling (Gerard 1982, Steneck and Graham 2003, Graham and Halpern 2004). Coastal upwelling, driven by Ekman transport of surface waters offshore, brings nutrient-rich waters from below the thermocline up to the photic zone where primary production can occur (Denny 2008). The influx of upwelled water during spring and summer months and its high concentration of nutrients, especially nitrate, fuels high primary production by giant kelp (Haines and Wheeler 1978, Gerard 1982, Zimmerman and Kremer 1986, Fram et al 2008). Seasonal changes in upwelling intensity affect the nitrogen content of kelp as well as its growth and survival (Gerard 1982, Graham et al. 2007). This study explores how mesograzer community structure relates to seasonal changes in kelp forests associated with coastal upwelling. Because of the high productivity of Macrocystis and mesograzers’ small size and energetic demands, it is unlikely that these species would be food-limited even when nitrogen is low and standing crop of kelp are reduced. However, upwelling-driven changes in available nitrogen may cause significant variation in the nutritional value of kelp with impacts on mesograzer growth and reproduction. In addition, the upwelling-driven changes in the growth of kelp plants themselves (Graham et al 2007) possibly affect these organisms through changes in available structural habitat.

The Hopkins Marine Life Observatory

The Hopkins Marine Life Observatory (MLO; see http://mlo.stanford.edu/) is the long- term monitoring program of Stanford University’s Hopkins Marine Station. MLO research examines change in the intertidal and nearshore habitats surrounding the Hopkins Marine Station in Pacific Grove, California. These systems have been protected from commercial disturbance since 1932, originally as part of the Hopkins Marine Life Refuge (1932-2005) and later the Hopkins State Marine Life Refuge (2005-2007) and Lovers Point State Marine Reserve (2007- present). Current MLO projects explore Hopkins’ coastal environment on all scales, addressing questions on topics that range from water quality and microbes to vertebrate populations. The MLO is currently executing a broad monitoring project on the local kelp forest community to investigate trophic interactions and seasonal patterns. As part of this research,

samples of kelp-associated invertebrates were taken over a 12-month period. The present study uses the exceptionally high-resolution time series of samples provided by the MLO to describe patterns in mesograzer community structure in relation to coastal upwelling. In the future, the trends identified here can be examined with respect to other MLO data and contribute to the characterization of larger-scale dynamics.

METHODS

Study site

This study examined the mesograzer community of the Hopkins Marine Station kelp bed, located off Pacific Grove, California (36.6205 ºN, 121.9045ºW). The kelp canopy at this site is composed entirely of giant kelp, Macrocystis pyriferia. Water depth is approximately 10m, and the kelp bed is partially protected from high wave action by its position northeast side of the Monterey peninsula. As part of the Lovers Point State Marine Reserve, this system does not directly experience commercial or recreational fishing pressures. All samples were taken from a 150 meter east-west transect located in the center of the kelp bed.

Sampling method

As part of the Marine Life Observatory program at Hopkins Marine Station, samples of kelp-associated invertebrates were taken regularly from August 2010 through July 2011. Every two weeks, samples composed of one frond were collected from six different Macrocystis plants along a permanent transect. All canopy zone kelp tissue of each frond and the organisms on or near its surface were gathered in a 250µm plankton net by MLO scientific divers. The canopy zone was defined as the portion of a frond extending from the water surface to a depth of approximately two meters; lower zones were not sampled. The plankton nets and kelp tissue from each separate frond were rinsed down thoroughly with fresh water over 250µm sieves to gather all invertebrates, which were then preserved in 10% formalin solution. Large snails (genus Chlorostoma) were the only organisms that were not preserved nor included in later analysis.

They were excluded because many escaped during sampling, rendering abundance data inaccurate. For each sample, the dry weight of kelp was also measured. Blade and stipe tissue were dried separately at 60°C for at least 24 hours and then weighed.

Sample Processing

The assemblage of canopy-associated invertebrates was quantified for all six preserved samples from one sampling date each month (Table 3). Processing for most of the samples (67 out of 72) was completed by the author, and the remaining were sorted by identical protocol. For each sample, organisms were separated into five size classes using sieves of mesh grades 250µm, 425µm, 850µm, 1mm, and 2mm. All specimens in the largest size class, >2mm, were measured to the nearest millimeter. All individuals were identified to the lowest possible taxonomic level via hand sorting under a dissecting microscope; no subsampling was utilized. After enumeration, organisms grouped by sample ID and size class were placed in 95% ethanol and are available for reference in further investigations.

Data analysis

The abundance data for all samples were standardized to units of individuals per gram of kelp biomass. Macrocystis biomass is highly correlated to surface area and therefore available mesograzer habitat, making it a useful denominator in reflecting the density of associated organisms (Coyer 1979). To normalize for kelp biomass, the total number of individuals in each taxonomic group and size class was divided by the summed dry weight of kelp blade and stipe tissue collected with the associated sample. Mesograzer densities found for the six samples from each collection date were averaged to approximate overall abundances. Multivariate analyses were carried out on the abundance data with the PRIMER-6 statistical package and PERMANOVA add-on (Clarke and Warwick 2001). Taxonomic community composition data was fourth-root transformed to down-weight the most abundant species, and Bray-Curtis dissimilarity between all pairs of samples was determined. Non-metric

Multi-Dimensional Scaling (MDS) determined multivariate similarity of samples, ANOSIM (analysis of similarity) tests detected differences between the collection dates, and SIMPER (similarity of percentages) analyses quantified the relative contribution of different taxa to observed dissimilarity of samples (Primer-6, Clarke and Warwick 2001). MDS, ANOSIM, and SIMPER analyses were also used to examine the relationship between coastal upwelling and community structure. The monthly upwelling index produced by Pacific Fisheries Environmental Laboratory (PFEL) for 36ºN, 122ºW (http://www.pfel.noaa.gov) was used as a proxy for coastal upwelling at the study site throughout the sampling period; index values varied between 1 and 222 (Table 3). Upwelling in a given month was categorized as high if the PFEL index value was above 150, medium if it was between 75 and 150, and low if it was less than 75 (Table 3). These thresholds were selected to evenly divide the range of observed indices into three groups. Community structure was evaluated in relation to the upwelling level of the month preceding sampling, as Macrocystis and the associated invertebrate community was expected to take several weeks before exhibiting any responses to changes in upwelling. The taxa contributing most highly to dissimilarity between high upwelling and low upwelling assemblages were identified with SIMPER, and their average abundances were calculated and compared. In addition to these categorical (high, medium, or low) upwelling analyses, the PFEL upwelling index’s relationship with the mesograzer community was also examined continuously with distance-based multivariate analysis for a linear model (DistLM) based on Bray-Curtis dissimilarity of samples. DistLM revealed the amount of variation between samples that could be explained by the PFEL upwelling index. DistLM was also applied to quantify the relationship between upwelling and variation in the size class structure of Idotea resecata, the largest taxon sampled. This analysis used a resemblance matrix of Euclidean distances between samples defined by untransformed size class abundance data. ANOSIM using the same Euclidean distance resemblance matrix tested for significant differences between sampling dates in Idotea populations.

RESULTS

Community composition and monthly change

Over the course of this study, 50,529 individual organisms were observed in the 72 samples (N=6 samples each month for 12 months). The most abundant group was the copepods (43,013 individuals, or 85.125% of the total; Fig. 1). Gastropods made up the next most abundant taxon (8.44% ). Gammarid amphipods (4.36%) were the third most prevalent, with isopods (1.07%), decapod shrimp (0.37%), and mysids (0.14%) following in abundance (Fig. 1). Larvae from various taxa combined totaled 0.2%. Genus and species-level distinctions were made when possible, and some taxonomic groups were represented almost entirely by members of a specific subgroup. Nearly all isopods were kelp isopod, Idotea resecata, and nearly all shrimp were the kelp humpback shrimp, Hippolyte clarki. Members of the genera Alia and Lacuna were the most common gastropods. The majority of gammarid amphipods observed were ampithoids, with the amphilochids following in abundance (Table 4). The dominant taxa varied in abundance between months. Copepods were most abundant in May, June, and July (Fig. 2a). Gastropods and gammarid amphipods reached their peak abundances in December and January respectively (Fig.2b). Isopods were also most abundant in December (Fig.2b). Copepods, gammarids and isopods each displayed only one strong peak during the sampling period, while gastropods had several notable maxima (Fig 2a, Fig.2b). The total species composition of the kelp mesograzer community varied significantly during the twelve-month sampling period (ANOSIM; R = 0.524, p = 0.001). Multidimensional scaling (MDS) visualized the higher relative similarity of samples collected on the same date (Fig. 5). MDS of monthly averaged abundance data also displayed the greater resemblance of communities sampled at similar points in the year (Fig. 7). Similarity of samples from the same collection date and dissimilarity of samples from different dates was determined for all months using similarity of percentage (SIMPER) analysis (Table 1). Pairwise comparisons showed that copepods, the most abundant taxon, were the top contributors to dissimilarity between sampling dates (SIMPER, Table 1).

Patterns in abundance by size class

Additional patterns emerged when size classes were examined separately (Figs. 3a-3e). While copepods dominated the smallest size class, none exceeded 2mm in length (Figs. 3a-3e). Numbers of the larger individuals increased as a whole in winter months, driven by an increase in amphipods in intermediate size classes and isopods (Idotea resecata) in the largest size classes (Fig 3b-e). I. resecata increased in both total numbers and size at this time of year (Fig. 4). ANOSIM analysis of isopod populations confirmed a significant difference in size class composition between sampling dates (R = 0.149, p = 0.001).

Community composition and upwelling

The variation in community composition between months was correlated with coastal upwelling (Pacific Fisheries Environmental Laboratory’s (PFEL) monthly upwelling index, Table 3). ANOSIM analysis indicated a significant difference between high and low upwelling periods (R=0.338, p= 0.001; significant for α of 0.017 with Bonferroni correction) defined by the PFEL index of the month preceding sampling. When comparing the average abundances of taxa for each collection date, upwelling explained 23% of the variability observed between different months (DistLM analysis; r2 = 0.23403, p = 0.003). Bray-Curtis dissimilarity was lower between the months in the same upwelling category (MDS, Fig 6, Fig 7). Months of high upwelling and low upwelling were the most distinct from each other, with a mean dissimilarity of 33.41% . The mean dissimilarities of high and medium upwelling months and medium and low upwelling months were 26.21% and 26.90%, respectively. Copepods contributed most to dissimilarity in all three comparisons despite fourth root transformation of raw abundances, which down-weights the importance of very abundant taxa. Along with copepods, Hippolyte shrimp, Lacuna and Alia snails, gammarid amphipods, and Idotea resecata exhibited variation in sampled abundances that drove dissimilarity between upwelling groups (Table 3). Patterns in size-class structure of the Idotea resecata population,

which had more large individuals in winter months (Fig 4), were additionally found to have a statistically significant correlation with upwelling levels (DistLM; r2 = 0.1357, p=0.001).

DISCUSSION

This study determined that the assemblage of kelp canopy mesograzers in the waters off of the Hopkins Marine Station changes between months. The most distinct seasonal patterns are an increase in the abundance of copepods in spring months and an increase in abundance and size of larger grazers, especially Idotea resecata, in the winter. These patterns correlate significantly with coastal upwelling, which is high in the spring and summer and low in fall and winter. The greater numbers of large-bodied mesograzers during periods of low upwelling suggests that grazing pressures are highest when kelp is also nutrient limited. The patterns characterized here may be examined further in relation to the wide body of continuous data available for this study site.

Community composition and monthly change

The mesograzer taxa identified in this investigation are consistent with those described in studies of other field sites (Jones 1971, Coyer 1979). However, the numerical dominance of copepods observed here distinguishes the Hopkins kelp canopy community from that of Habitat Reef at Santa Catalina Island, which contains more gammarids and fewer copepods, gastropods and isopods (Coyer 1979). Since the assemblage at Habitat Reef has not been recently described, however, year-to-year or longer-term variation may be a factor in these differences. Our use of a 0.25 mm plankton net rather than a 0.33 mm net, as Coyer (1979) used, may have inflated the copepod abundance measured here, though high numbers of copepods greater than 0.33 mm observed show that alone does not account for the different abundances. The higher abundance of copepods in the Hopkins kelp canopy may drive differences in the diets of juvenile fish between this site and Habitat Reef (Coyer 1979, Singer 1985).

The Hopkins seasonal pattern of copepod abundance, with a strong peak in spring, also differs from Coyer (1979) who found high variability in copepod abundance with no discernable trends. Here the large spring peak in copepod numbers was the strongest driver of the variability between sampling dates (Fig. 2). That copepods were the top contributor to dissimilarity between months in 27 of the 66 SIMPER pair-wise tests underlines this fact (Table 2).

Patterns in abundance by size class

While much of the variation in abundance determined by multivariate analysis was driven by copepods, further patterns in the mesograzer community became apparent when size class data were examined. Since in larger size classes there is more biomass per individual, smaller changes in their abundances have greater implications for the distribution of mesograzer biomass in the kelp forest community. Mesograzer biomass totals were not calculated in this study, as length-to-weight conversion regressions from the literature could not be used. The biomass regressions Coyer (1979) applied for both gammarid amphipods and the isopod Idotea resecata were determined using individuals substantially smaller than some individuals found here. These groups visually appeared to be the most significant contributors to mesograzer biomass in many samples, and so biomass calculations excluding them would be misleading. Data from this study will be converted to biomass in the future with the use of newly determined length-to-weight regressions; the sampled individuals cannot be directly weighed, as preservation has variable effects on mass. Despite the lack of biomass measures, Figs. 3c-3e show increases in large individuals over winter months that suggest there is greater mesograzer biomass at that time, even though the greatest numerical abundance is in May and June (Fig. 2). The ecological implications of this winter spike in large isopods (16-24mm and 25-32mm, Fig. 5), as well as amphipods, gastropods, mysids, and shrimp are discussed below in relation to other seasonal dynamics.

Community composition and upwelling

1. Overall effects Variation in the canopy invertebrate community is correlated with upwelling. Since upwelling co-varies with many other seasonal factors, the significant ANOSIM and DistLM results alone cannot determine if differences between samples in the high and low upwelling groups are caused by other environmental conditions. There were many sources of variability affecting the composition of the samples evaluated in this study; environmental factors ranging from predatory fish abundance to water motion likely have complex effects on the mesograzer community (Coyer 1979, Foster and Shiel 1985, Duggins et. al 2001, Davenport and Anderson 2007, Perez-Matus and Shima 2010), and sampling itself inevitably introduces more variation in measured abundances. In accounting for 23% of this variability, however, upwelling was shown to be a useful predictor of the mesograzer assemblage. It may have a causal role in the differences in mesograzer abundance between months that follow high versus low upwelling. Upwelled nutrients can cause shifts in the nutritional quality of invertebrates’ food sources and the amount of available complex algal habitat with effects on survivorship and reproduction of invertebrates (Jormalainen and Honkanen 2008, North 1971a).

2. Copepod abundance The greatest contributor to dissimilarity between high and low upwelling samples was copepods, the same primary driver of between-month variability (Table 4). The kelp-associated copepods, which were nearly all harpactacoid species (primarily Tisbe spp. and Porcellidium spp.), had a mean abundance approximately four times greater following high versus low upwelling periods, though variability was high (Table 4). The greater abundances during spring and summer were consistent with seasonal peaks in epiphytic harpacticoid copepod populations described by Hicks (1977a) and Webb and Parsons (1992). This peak in abundance of kelp- associated species may counter a decline in nearshore planktonic copepods that are advected offshore with surface waters during periods of upwelling (see Peterson 1998). Webb and Parsons (1992) suggest that the spring “bloom” commonly seen in epiphytic harpacticoids, which feed on algal biofilms (Hicks 1977c), is related to seasonal increases in

bacterial and microalgal production. It is possible that the changes in copepod density are related to increases in these biofilms’ nutritional quality caused in part by upwelling; food supply influences copepod fecundity and growth rates and therefore can cause an increase in standing population sizes (Hicks 1977b). Hicks (1977b) notes that food may not be seasonally limiting for phytal copepods found in shallow waters, however, which would rule out the possibility of direct bottom-up effects on harpacticoid populations. Additionally, the abundance of bacterial biofilms is lower in the Hopkins kelp forest in spring and summer (Lee and Michelou, unpublished data). Seasonal abundances and grazing rates of predatory fish may be more important influences on copepod population changes through top-down mechanisms (Coyer 1979, Foster and Shiel 1985, Davenport and Anderson 2007, Perez-Matus and Shima 2010). Preliminary MLO fish abundance data indicates higher numbers of juvenile rockfish (genus Sebastes), likely predators of these copepods, in late summer and early fall (Litvin, unpublished data). Upwelling may therefore affect copepod numbers with a longer lag than examined in this study through influencing rockfish recruitment. Juvenile Sebastes populations are correlated to upwelling, which is a better predictor of their settlement than either sea surface temperature or Chlorophyll-a levels (Caselle 2010).

3. Kelp density effects Before variation in mesograzer community is further discussed, an important point on their abundances must be addressed. While comparing abundances over this time series and across the upwelling groups, one must consider the distinction between changes in absolute populations of invertebrates within the kelp bed and changes in the concentration alone of invertebrates, as individuals were measured per unit of kelp biomass or surface area. In Coyer (1979), Macrocystis density remained constant over most of the study period at his Habitat Reef field site, and the patterns he described in mesograzer abundance in units of individuals per kilogram kelp biomass can therefore be assumed to represent changes in absolute abundance of invertebrates (Coyer 1979, Coyer 1984). As kelp density data were not incorporated in the present study, however, there is a possibility that invertebrate abundance patterns do not reflect changes in absolute populations of mesograzers but rather only their concentration on standing kelp. The Hopkins Marine Station kelp bed, along with various other study sites (Graham et al.

1997, Duggins et al. 2001, Steneck and Graham 2007), loses canopy mass in the winter due to attrition, storm damage, and slower addition of new fronds (Watanabe & Phillips, unpublished. data). Therefore, it is likely that this reduction in kelp surface area plays a role in the greater abundance numbers of some taxa in the winter month samples, as more individual invertebrates aggregated to fewer attached plants. The irregular size class succession patterns of Idotea resecata, as shown in Figure 5, suggest that dynamics other than changes in total population such as migration from kelp tissue detached in storms may effect changes in the sampled community.

4. Abundance patterns of larger grazers The kelp-associated invertebrates that use kelp tissue itself as a major food source, including Idotea, Lacuna, and gammarid amphipods (Jones 1971), are more abundant following low upwelling (Table 3). This winter increase in larger mesograzers cannot be easily explained by direct effects of upwelling. Increased food quality and subsequent growth and reproduction do not seem to be a cause of greater sizes and abundances. Since Macrocystis can store nitrogen for no longer than 30 days (Gerard 1982), kelp exhibits reduced nitrogen content in the winter when fewer upwelled nutrients are available and is therefore less nutritious during this period. The winter reduction of standing kelp and resulting concentration of invertebrates to fewer kelp blades may be the primary driver behind the higher low-upwelling abundances seen in the present study. Whether or not this is the case, Macrocystis nonetheless faces higher grazing pressures per unit area during times that upwelling has been low. This is especially true regarding grazing by Idotea resecata, which are larger and more abundant in winter months of low upwelling (Fig. 3b-e, Fig. 4). This finding has important ecological implications for kelp forest dynamics. Higher grazing specifically in times following low upwelling may result in feedbacks of further grazing damage and kelp tissue loss, increasing the influence canopy invertebrates have on Macrosystis and associated organisms. Low nitrogen content of kelp tissue, typical of low upwelling periods, has been correlated with higher grazing rates by mesograzers as they compensate for lowered food quality (Cruz-Rivera and Hayes 2003). Grazing rates also increase with water temperature, which is higher when upwelling is relaxed (Leighton 1971). Both of these factors could predispose kelp to higher grazing rates during low-upwelling periods. Low

ambient nitrogen reduces overall growth rates, limiting the kelp’s ability to compensate for tissue lost to increased grazing as well (North 1971a, Gerard 1982, Zimmerman and Kremer 1986). Specific attributes of Idotea resecata’s grazing behavior further amplify the deleterious effects it may have on Macrocystis. I. resecata preferentially grazes at the bottom of Macrocystis blades, near the point where the blade is connected to the pneumatocyst (gas float) and stipe (Jones 1971). This is the thickest part of the blade and richest in carbohydrates, as nutrients for the entire blade travel through this area (Jones 1971). The intensified grazing close to the blade’s point of attachment makes the blade vulnerable to severance from the kelp frond much earlier than it would be in the absence of grazing (Jones 1971). This results in a high rate of tissue loss for kelp relative to the amount of tissue actually consumed. Blades that become detached following isopod grazing are often exported from the canopy, either sinking or floating away, depending on whether the point of attachment was above or below the pneumatocyst and entanglement in nearby kelp (Jones 1971). In this manner, grazing by relatively small Idotea can affect whether organic material is retained or exported from the kelp forest environment. The kelp tissue lost as a result of isopod grazing also may have important effects on the kelp’s ability to take up nutrients. In Macrocystis, canopy blades have very large roles in the plant’s total nitrogen uptake, exhibiting uptake rates approximately 60% faster than blades at greater depths (Gerard 1982). These canopy blades are lost due to grazing by isopods at much higher rates than blades positioned deeper down, as Idotea are generally restricted to the canopy zone (Jones 1971, Coyer 1979). Idotea may therefore reduce algal growth by indirectly suppressing nitrogen uptake, an effect Bracken and Stachowicz (2007) suggested the kelp crab, Pugettia producta has on the algae Egregia menziesii by reducing its proportional surface area. Idotea are smaller and less intense grazers than the kelp crab; however, the fact that Idotea numbers and sizes are greatest during periods following low upwelling could amplify an otherwise small effect, as Macrocystis is already experiencing nutrient shortages in these times. In addition to directly consuming kelp tissue, contributing to its loss through breakage, and potentially reducing nitrogen uptake and subsequent growth of kelp, isopods also affect Macrocystis by damaging its tissues and leaving them vulnerable to other injury. Puncture wounds that Idotea’s sharp claws leave in Macrocystis blades along with damage caused directly by grazing contribute to bacterial and fungal infection of the alga (Jones 1971). Therefore, even

if they did not eat kelp tissue, isopods’ high winter concentrations could still have a detrimental effect on Macrocystis at the same time it confronts lower ambient nutrients and shorter day lengths. Considering all the mechanisms in play, it is not surprising that these smaller-bodied grazers have been found capable of denuding entire kelp bed canopies (Foster and Schiel 1985). This interspecies interaction would likely be much less powerful if isopod numbers peaked during high-upwelling spring and summer months, when they are typically low (White and Clendenning 1971, Coyer 1979). It is also possible, though, that the standing kelp in winter is not as strongly affected by damage from grazing as it might initially appear. Because many kelp fronds present in winter are growth from the summer, more are relatively old and closer to senescence, as maximum frond lifespans are about six months (North 1961). Even if this is the case and mezograzers primarily damage kelp that is already degrading during low upwelling, their grazing may still be important in its facilitation of tissue loss from the canopy. Detached kelp tissue is an important source of organic material for systems such as beaches and ocean trenches (Harrold et al. 1998). Additionally, expedited removal of dying canopy tissue may be influential within the kelp forest in increasing light availability and encouraging turnover with the growth of new fronds. Experimental studies that examine whether isopod grazing in winter reduces Macrocystis biomass, and whether that biomass is mostly functional or degraded, are needed to discern how much these mesograzers contribute to the seasonal stresses faced by kelp.

Conclusions

The mesograzer abundance data observed over the course of this time series, though variable, exhibited clear seasonal patterns. Spring peaks in copepods and winter peaks in larger organisms were distinct. These shifts in mesograzer community structure were correlated with upwelling. While causal relationships cannot be discerned conclusively, the correlation between mesograzer community composition and upwelling may have interesting ecological implications both for the dynamics of mesograzer populations and for the life histories of the Macrocystis plants these invertebrates are associated with. To better understand the effect of mesograzer populations on the kelp forest community at different points of the year, further laboratory and in

situ investigations are needed. Mesograzer predator exclusion studies could offer insight into whether these invertebrates have strong, predator-mediated effects on kelp in central California kelp forests, as they do in southern regions (Davenport and Anderson 2007). Examinations of Macrocystis nitrogen content and its effect on mesograzers’ growth and reproduction could reveal whether upwelling has any direct bottom-up effects on the kelp-associated vertebrate assemblage. Additionally, longer-term monitoring of the mesograzer community, especially over years with anomalous upwelling such as ENSO events, could reveal how strong a predictor upwelling is for the abundances of different mesograser groups, such as copepods and isopods, and their individual influences on kelp forest food webs. Kelp forests are complex systems rich in biodiversity. However, aerial photograph surveys suggest that the total area of kelp forest canopy along the California coast has declined over the past several decades (Leet et al. 2001). These habitats are also vulnerable to harmful changes in coastal upwelling’s magnitude, location, and frequency that accompany climate change (Hardley et al. 2006). In order to effectively preserve healthy kelp forests and the services they provide, it is important to grasp the complex connections between of all their biological players. The inconspicuous mesograzer community is dynamic and full of diverse characters. The more fully its role in kelp forests is understood, the better it can be examined as an indicator of total system flux and stability.

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Appendix

Table 1. SIMPER results. Top half of table shows average dissimilarity of individual samples from the two months as a percent. The taxon accounting most highly for dissimilarity between the two dates is shown in the lower month-to-month intersections. Bold values indicate average similarity percentage among samples of the given sample date.

PFEL Sample PFEL Monthly Collection Upwelling Index Date Date Index Range

15-Jul-10 --- 196 High 15-Aug-10 25-Aug-10 189 High 15-Sep-10 13-Sep-10 105 Medium 15-Oct-10 20-Oct-10 27 Low 15-Nov-10 18-Nov-10 37 Low 15-Dec-10 15-Dec-10 1 Low 15-Jan-11 25-Jan-11 22 Low 15-Feb-11 23-Feb-11 18 Low 15-Mar-11 22-Mar-11 23 Low 15-Apr-11 28-Apr-11 180 High 15-May-11 31-May-11 198 High 15-Jun-11 28-Jun-11 203 High 15-Jul-11 29-Jul-11 222 High

Table 2. Monthly Pacific Fisheries Environmental Laboratory (PFEL) upwelling indices for 36ºN, 122ºW (study site is located 36.6205 ºN, 121.9045ºW) during sampling period. Each month was categorized as high upwelling if its index value was above 150, medium if it was between 75 and 150, and low if it was less than 75.

High SD SD (All Low SD SD (All Species Upwelling (Monthy) Samples) Upwelling (Monthly) Samples) copepod 45.874 35.274 47.360 11.387 4.279 8.658 Hippolyte 0.014 0.024 0.037 0.224 0.186 0.403 Lacuna (and Eulithidium) 1.066 0.830 1.227 2.464 2.381 3.700 gammarid 0.690 0.402 0.853 2.308 2.940 4.525 Alia 0.768 0.787 1.032 0.529 0.385 0.492 Garnotia* 0.159 0.086 0.135 0.084 0.066 0.116 Idotea resecata 0.168 0.068 0.180 0.626 0.597 0.863 cyprid 0.031 0.032 0.064 0.075 0.123 0.134 mysid 0.046 0.033 0.110 0.060 0.067 0.113 ostracod 0.047 0.037 0.065 0.022 0.033 0.066 caprellid 0.004 0.005 0.015 0.042 0.089 0.177 bivalve 0.008 0.017 0.024 0.018 0.033 0.051 Unknown larva 0.001 0.003 0.027 0.008 0.017 0.000 urchin 0.004 0.008 0.014 0.010 0.017 0.043 Table 3. Abundance of taxa (individuals per gram kelp biomass) that contributed most highly to dissimilarity between samples from the high upwelling group (PFEL upwelling index of preceding month >150) and low upwelling group (PFEL upwelling index of preceding month <75). For each taxon, its higher abundance measure (between the two upwelling categories) is shown in red. Taxa are listed in descending order of relative contribution to dissimilarity as determined by SIMPER analysis. *Garnotia, the hitchhiker limpet, is usually found attached the larger gastropods that were imprecisely sampled and excluded from this analysis; its measured abundances may therefore be imprecise as well.

Gastropods Crustaceans Echinoderms Miscellaneous

bivalve Copepod juvenile asteroid Hirudinid worm juvenile urchin decapod shrimp (genus Strongylocentrus Lacuna and Eulithidium Hippolyte) franciscanus clingfish genus snail and/or S. purpuratus cryptic small vermetid snail caprellid amphipod unknowns gammarid amphipod (Ampithoe plea; Perampithoe humeralis; Hyale Alia genus snail frequens; Ampithoe spp;, unidentified amphiloids; and unidentified gammarids, general) Melibe leonina Idotea resecata isopod nudibranch

unknown dorid Idotea schmitti isopod nudibranch

unknown eolid sphaeromatid isopods nudibranch

Garnotia adunca snail decapod larva: zoea

decapod larva:

megalope

barnacle larva: cyprid

Mysid

Ostracod Table 4. Full list of taxa identified in the completion of this study. For every sample, the abundance of each group noted above was determined and applied in multivariate analysis of community composition.

FIGURES

Figure 1. Total individuals observed in all samples by broad taxonomic group.

Figure 2a. Monthly abundances of the most common taxonomic groups. Values were determined by averaging the abundances observed in the six samples from each collection date.

Figure 2b. Monthly abundances of common taxonomic groups excluding copepods. Leaving out copepods shows trends in less abundant taxa. Values were determined by averaging the abundances observed in the six samples from each collection date.

Figure 3a. Monthly abundances for individuals of common taxa less than 2mm in length. Values were determined by averaging the abundances observed in the six samples from each collection date.

Figure 3b. Monthly abundances for individuals of common taxa between 2mm and 6mm in length. Values were determined by averaging the abundances observed in the six samples from each collection date.

Figure 3c. Monthly abundances for individuals of common taxa less between 6mm and 11mm in length. Values were determined by averaging the abundances observed in the six samples from each collection date.

Figure 3d. Monthly abundances of individuals of common taxa less between 11mm and 20mm in length. Values were determined by averaging the abundances observed in the six samples from each collection date.

Figure 3e. Monthly abundances for individuals of common taxa greater than 20mm in length. Values were determined by averaging the abundances observed in the six samples from each collection date.

Figure 4. Uniformly scaled size class histograms of Idotea resecata for each month in the sampling period. Size classes are in millimeters, each including an 8mm range.

Figure 5. MDS plot (Bray-Curtis dissimilarity) of all samples labeled by the month of collection.

Figure 6. MDS plot (Bray-Curtis dissimilarity) of all samples, with the upwelling group (high, medium, or low) of the month preceding their collection indicated.

Figure 7. MDS plot of Bray-Curtis similarity of monthly averaged community composition data. Upwelling group of the month preceding the month of sample collection is indicated by color and chronology (beginning with August) by lines.