Estuaries and Coasts (2016) 39:248–257 DOI 10.1007/s12237-015-9984-9
The Influence of Stickleback on the Accumulation of Primary Production: a Comparison of Field and Experimental Data Ulrika Candolin 1 & Anna Johanson 1 & Alexandre Budria 1
Received: 28 August 2014 / Revised: 4 May 2015 / Accepted: 5 May 2015 / Published online: 15 May 2015 # Coastal and Estuarine Research Federation 2015
Abstract Humans are currently altering aquatic ecosystems through overfishing and nutrient loading. This is altering ecosystems through top-down and bottom-up processes. In the Baltic Sea, the removal of top predators has increased the density of mesopredators, while eutrophication has boosted primary production. Using a mesocosm experiment, we show that a high density of a mesopredator—the threespine stickleback Gasterosteus aculeatus—increases the biomass of primary production not transferred to higher trophic levels under simplified ecosystem conditions. However, no effect of stickleback density on algae biomass is detected in the field, although stickleback gut inspection reveals that stickleback are feeding on amphipods, and stickleback density tends to correlate negatively with amphipod density in the field. This suggests that trophic cascades induced by the mesopredator release are attenuated in the field and do not reach primary producers. This is probably caused by the complexity of the ecosystem where many processes regulate the food web, such as bottom-up effects, the presence of alternative prey and density-dependent predation and distribution. However, the degree to which the ecosystem will be able to buffer further changes, if human disturbances continue, is unknown. Trophic cascades that reach primary producers could have drastic consequences for the ecosystem by promoting the accumulation of drifting algal mats that alter the benthic habitat.
Communicated by: Charles Simenstad * Ulrika Candolin
[email protected] 1
Department of Biosciences, University of Helsinki, P.O. Box 65, 00014 Helsinki, Finland
Keywords Algal mats . Eutrophication . Filamentous algae . Grazers . Mesopredator release . Trophic cascades
Introduction Primary production has increased dramatically in many aquatic ecosystems because of human-induced eutrophication (Smith 2003). This could influence the flow of material and energy through the ecosystem (e.g. Korpinen et al. 2010; Kraufvelin et al. 2010; Marczak et al. 2007). The increased production could be transmitted to higher trophic levels and extend the length of the food chains or be transferred directly to decomposers and amplify the amount of decaying primary production, with further consequences for the ecosystem (Cloern 2001; Valiela et al. 1997). In an ecosystem in balance, species at higher trophic levels regulate the biomass of organisms at lower trophic levels through density-dependent predation and herbivory (Hixon et al. 2002; Murdoch 1994). However, this regulation can be disrupted if primary production suddenly increases and higher trophic levels are not able to keep up with the increase (Fox et al. 2012). This may be the case when species at higher trophic levels have not experienced rapid boost in primary production in the past and, hence, have not evolved adaptive reaction norms for responding to such changes (Lande 2009; Michel et al. 2014; Reed et al. 2010). Moreover, changes in the abundance or behaviour of top predators can disrupt the regulation of primary production (Estes et al. 2011; Svensson et al. 2012). In particular, human activities such as overfishing that reduce top predators can induce trophic cascades that increase the biomass of mesopredators, which, in turn, can reduce the biomass of herbivores (Carpenter et al. 2001; Pace et al. 1999; Worm et al. 2002). The end result can be
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drastic increases in the accumulation of decaying primary production. A growing number of studies demonstrate the influence of changes in the abundance or behaviour of top predators on food webs and the structure and function of aquatic ecosystems (e.g. Dalton et al. 2013; Estes and Duggins 1995; Heithaus et al. 2008; Schmitz et al. 1997). However, the mechanisms behind these top-down effects are often complex, as ecosystems consist of a multitude of interacting factors (Estes et al. 2011; Reynolds and Bruno 2013; Rooney et al. 2006). In the Baltic Sea, humans have altered the ecosystem by increasing nutrient input and by removing top predators (Bonsdorff et al. 1997; Ljunggren et al. 2010). This has enhanced primary production and induced a mesopredator release, i.e. an increased density of medium-sized fishes (Casini et al. 2008; Ljunggren et al. 2010; Osterblom et al. 2007). The increased primary production has in turn reduced water transparency, increased sedimentation rate, promoted the build-up of mats of drifting filamentous algae—which consume dissolved oxygen and cause hypoxia —and altered the composition of the species community (e.g. Berglund et al. 2003; Cloern et al. 2014; Korpinen et al. 2007; Norkko and Bonsdorff 1996; Sanden and Hakansson 1996). The mesopredator release has again been hypothesized to amplify the biomass of primary production by decreasing the biomass of grazers and, thereby, the consumer control of primary producers, in line with the prediction of trophic cascades (Eriksson et al. 2009). However, mesopredators are often omnivorous and consume not only herbivores but also primary producers and carnivores, such as early life history stages of fishes (Bruno and O’Connor 2005; Eriksson et al. 2011; Neutel et al. 2002). The release of omnivorous mesopredators could consequently weaken human-induced trophic cascades and prevent them from reaching primary producers or, alternatively, reinforce trophic cascades and further promote the accumulation of primary production by feeding on early life history stages of larger piscivorous fishes. An abundant omnivorous mesopredator in many coastal ecosystems of the Northern Hemisphere is the threespine stickleback, Gasterosteus aculeatus. It feeds on macroinvertebrates, zooplankton, detrivores and early life history stages of meso and top predators (Wootton 1976). It is consequently an important component of numerous coastal food webs. In the Baltic Sea, stickleback populations have grown during the last decades, possible because of a human-induced reduction in the density of apex predators (Ljunggren et al. 2010). At the breeding sites of the stickleback in the Northern Baltic Sea, which usually are rocky shore beds, densities of adult stickleback have increased also because of a rampant growth of algae that reduces visibility and allows more males to nest within an area (Candolin 2004; Candolin et al. 2008, 2014). The influence of an increased abundance of adult stickleback on the accumulation of primary production has been the
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focus of several studies. Sieben et al. (2011) found an experimental increase in the density of stickleback to shift the composition of grazers from amphipods to gastropods but detected an impact on the biomass of macroalgae only under artificially high nutrient levels and not under natural levels. The influence of stickleback on phytoplankton in a lake enclosure experiment was again evident only at high fish densities: Jakobsen et al. (2003) showed experimentally that only above densities of four to six stickleback per square meter are zooplankton abundance reduced and phytoplankton turbidity increased. Likewise, Des Roches et al. (2013) in a mesocosm experiment found the presence of stickleback to promote the growth of phytoplankton only under high fish densities and to have no effect on larger zooplankton. Thus, the influence of stickleback on the biomass of primary production appears to be weak under natural conditions. However, the influence, or the tipping point at which effects become apparent, could vary among ecosystems, depending on the composition and structure of the food web and the degree to which stickleback can switch between alternative food sources. We investigated if the current mesopredator release of stickleback in the Northern Baltic Sea influences the accumulation of filamentous algae at stickleback breeding sites where the density of stickleback is higher than in surrounding areas. We combined an experimental manipulation of stickleback density under simplified mesocosm conditions with a correlative field study to determine (1) the degree to which stickleback are consuming grazers, (2) if this influences the biomass of algae under mesocosm conditions when only the presence of stickleback is manipulated and (3) if an effect of stickleback is apparent in the field where many factors are influencing the food web. Mesocosm experiments tease out the influence of one factor on another but may not reveal processes acting in the field, where factors not included in the experiment may override the experimentally detected effects. The combination of field studies with experimental work can then be informative in disentangling the importance of various factors and processes regulating ecosystems under natural conditions.
Methods Stickleback Diet Analysis To determine the degree to which stickleback are feeding on grazers during the breeding season, we caught threespine stickleback from four breeding sites along the coast of southern Finland (Fig. 1): Rauma (61° N, 21° E), Nauvo (60° N, 21° E), Tvärminne (60° N, 23° E) and Kotka (60° N, 27° E). The sampling was done in mid-May, mid-June and mid-July in 2011 and additionally from Tvärminne in June and July 2013. All sites have clear water, and the vegetation consists mainly of filamentous algae.
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Fig. 1 The investigated sites in Southern Finland
The stickleback were caught using hand nets and minnow traps, killed through decapitation and either frozen (fish caught in 2011) or preserved in 70 % ethanol (fish caught in 2013). The gut content of individuals >4 cm was inspected under a stereomicroscope and the remains of organisms determined to species, family or order level, depending on the quality of the remains. The number of individuals of each organism group could not always be determined, and we therefore noted the presence or absence of representatives of identified organism groups. We used generalized linear mixed models (GLMM) with binomial probability distribution to analyze the presence of amphipods, the only common grazers in the guts. We inserted year and site as random factors, with site nested within year, time as fixed factor and the presence or absence of amphipods as the bivariate response variable. The software used (in this and all other analyses) was IBM SPSS Statistics 20. Mesocosm Experiment We collected live filamentous algae Cladophora glomerata with associated invertebrate fauna from rocky shores of the Tvärminne region in early June 2013. The algae were detached from the rock with a knife, transported to the station and kept in an oxygenated tank. About 60 g of the algae were attached to each of 20 plastic nets (60×30 cm, mesh size 2 mm) through sewing. The nets were placed into a large pool, kept in an outdoor facility under natural light and temperature conditions, for 5 days to homogenize the distribution of the invertebrate fauna. The 20 nets (with C. glomerata and invertebrates) were placed into individual 30-l plastic tanks, one net into each tank. Small stones were placed on top of the nets to keep them in place. The tanks held 25.5-l of water and were kept under natural light and temperature conditions in the outdoor facility. Sea water was slowly flowing through the tanks (50–150 ml/ min), and a net at the outflow (mesh size 0.2 mm) prevented organisms from escaping the tanks. Ten additional Gammarus sp., size about 10 mm, were added to each tank, as larger amphipods could have escaped when collecting and transferring the filamentous algae. The exact number of amphipods in each tank was unknown as the number added with the nets was unknown.
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To half of the tanks, we added a stickleback, 55–70-mm standard length (mean±SE: 65±1 mm), which corresponds to a high density of five adult stickleback per square meter. The other ten tanks served as controls. The stickleback were caught with minnow traps in early June and kept in large holding tanks until being used in the experiment. They were fed defrosted chironomid larvae daily (in the holding tanks, not during the experiment) and were in good condition. Breeding males, recognized by their blue eyes, were not used in the experiment, as these could establish territories, which could influence their feeding behaviour. The experimental tanks were checked at least once a day. Stickleback that died were replaced by stickleback of the same size. Mortality is naturally high in July when the breeding season ends (Candolin 2000; Candolin et al. 2014). Five stickleback died and were replaced. The experiment ran from 13 of June to 14 of July in 2013. After the 31 days, we removed the stickleback, detached the filamentous algae from the nets and collected all invertebrates visible to the eye and stored them in 70 % ethanol for species or family determination. To analyze the intensity of the green colour of the filamentous algae, we photographed 50 g of the algae in petri dishes under standardized light conditions, against a white background. The colour depends on the overgrowth by epiphytic diatoms and the production and degradation of the chlorophyll in the cells (Power et al. 2009; Zulkifly et al. 2013). To analyze the colour from the digital images, we used image analysis software (IMAGEJ v. 1.47) and the plugin ‘Measure RGB’. We first selected the area of the petri dish and then measured the green colour as G/(G+R+B). To determine algae dry weight, the algae were dried at 70 °C for 3 days. To determine dry weight of epiphytes, these were scraped from the walls of the tanks and the small stones, sieved through a 0.5-mm sieve and dried at 70 °C for 3 days. We used linear models to analyze the effect of stickleback on the number of amphipods left at the end of the experiment, changes in the wet weight of C. cladophora and epiphytes during the experiment and the colour of C. cladophora at the end of the experiment. Correlative Field Investigation We monitored the densities of threespine stickleback, filamentous algae and invertebrate grazers in three shallow coastal bays in the northern Baltic Sea: Långskär bay near Tvärminne Zoological Station (60° N, 23° E), which has a very high density of stickleback during the breeding season, Marviken at Stora Herrö in Espoo (60° N, 25° E), which has a moderate density of stickleback, and Herröviken in Espoo (60° N, 25° E), which has a low density of stickleback (Fig. 1). Långskär bay in Tvärminne was monitored from 3 June to 25 July in 2013 and Marviken and Herröviken in Espoo from 31 May to 28 July in 2013.
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All sites have stony seabeds overgrown with ephemeral filamentous algae, mainly the green algae C. glomerata, and occasional stands of bladderwrack Fucus vesiculosus. Nutrient levels are slightly higher in the waters outside the sites in Espoo than in Tvärminne (Table 1). Common predators on grazers, in addition to threespine stickleback, are European perch Perca fluviatilis; ninespine stickleback Pungitius pungitius; minnow Phoxinus phoxinus; and the shrimps Crangon crangon, Palaemon adspersus and Palaemon elegans. Common predators on threespine stickleback are terns, Sterna hirundo and Sterna paradisaea, European perch and Northern pike Esox lucius. Density of Stickleback The density of stickleback was monitored once a week between 9 and 15 h. We selected five 2-m2 observational squares (length along coast 2 m) at a depth of 0.2–1.0 m at each of the three sites. The corners were marked using floats attached to pieces of brick. We observed the squares from the rocky shores during 10 min, counting the number of threespine stickleback larger than about 2 cm, thus excluding juveniles born in the same year that are markedly smaller than older stickleback. Threespine stickleback are easily identified based on their morphology, movement pattern and coloration. We noted whether the observed fish were nuptially coloured males. We used GLMM with log-link function to analyze changes over time in stickleback density, with number of stickleback observed in each square as the response variable, site as random factor and both time and (time)2 as fixed factors to analyze for both linear and curvilinear relationships. To investigate differences between sites in stickleback density, we inserted site as fixed factor (instead of as random factor) in the model. Biomass of Algae and Abundance of Grazers We sampled filamentous algae and the grazer community at each site immediately after recording stickleback density. We selected five rocky spots with flat surfaces where samples could be taken (samples could not be taken over uneven Table 1 Nutrient levels in surface waters outside the investigated bays in Tvärminne and Espoo in 2013 Site
Ptot (μg/l) Ntot (μg/l)
Tvärminne (Storfjärden)
Espoo (Pentala)
21 May
13 June
9 July
15 May
21 August
25 363
16 316
17 307
29 410
26 420
The data is compiled from the web portal of the Finnish Environmental Administration (http://wwwp2.ymparisto.fi/script/oiva.asp) and from monitoring data collected by Tvärminne Zoological Station
surfaces) at a depth of 0.2–0.5 m. Within each rocky spot, we selected randomly where to sample. We placed a net bag (mesh size 0.2 mm, net length 15 cm) attached to a metal ring (diameter in Långskär 14 cm, total area 154 cm2, diameter in Marviken and Herröviken 13.5 cm, total area 143 cm2) against the rocky surface and cut off the algae with a sharp knife. We closed the net around the algae and lifted the algae mass into a bucket with sea water. Some invertebrates could have escaped during handling, but the error should be the same across replicates. At each sampling date, we selected different sampling spots within the sites. In the lab, we separated the animals from the algae under a stereomicroscope and identified individuals >0.5 mm to species or family level. The grazers were divided into three groups: gastropods, amphipods and isopods. The algae were dried at 70 °C for 3 days to determine dry weight (dwt). We used linear mixed models (LMM) to investigate the influence of time on the abundance of grazers and macroalgae, with site as random factor and time as fixed factor. To analyze for non-linear relationships over time, a quadratic time term (time)2 was added to the model. To analyze for differences between sites, site was inserted as a fixed factor. To analyze for correlations between stickleback density and grazer and algae density, the density of stickleback was inserted as a covariate into the models, using average values for each time point and site. The distribution of the error term was checked and the variables transformed when needed to meet the assumptions of the tests. Gastropod and amphipod density and number per gram dwt algae were square-root-transformed.
Results Stickleback Diet Analysis Across time and sites, 13 % of stickleback (N=382) had remains of amphipods in their guts, with the proportion decreasing over the season (F3,379 =10.23, P
