Mussels and canopy-forming algae as ecosystem engineers : their contribution to community organization in the rocky sublittoral

Understanding the influence of biogenic habitats on species assemblage structure and ecosystem productivity is crucial for successful conservation of natural systems. Brown algae, Fucus vesiculosus, and blue mussels, Mytilus trossulus, coexist on sheltered and moderately wave exposed shallow rocky reefs of the northern Baltic Proper. Here, they function as important biogenic structures for an abundant associated macroinvertebrate fauna. Despite their dominance and space sympatry, there is little understanding of how they differ in their role as provisioners of biodiversity in this system. While Fucus has been recognized as an important habitat provider for decades, the similar role of blue mussels has been seriously understudied in the northern Baltic Proper, leading to pressing knowledge gaps and an underestimation of their role for overall biodiversity. In this study, we compared macroinvertebrate species assemblages within 40 rocky reefs where Fucus and Mytilus co-occur in either intermixed or adjacent assemblages. We show that both habitats represent a species rich and abundant community that are comparable regarding diversity. However, abundance and biomass of the associated community is much higher in the Mytilus habitat in relation to the Fucus habitat, implying a far higher secondary production in the former habitat. Recognizing key habitats and understanding how they differ in their ability to support biodiversity and ecosystem productivity is necessary for predicting community responses to human pressures, including an altered climate, and for implementing efficient mitigation actions to minimize loss of biodiversity.Peer reviewe

Blåmusslans populationsdynamik i en varierande miljö vid randen av artens utbredning

Spatial and temporal variation in the abundance of species can often be ascribed to spatial and temporal variation in the surrounding environment. Knowledge of how biotic and abiotic factors operate over different spatial and temporal scales in determining distribution, abundance, and structure of populations lies at the heart of ecology. The major part of the current ecological theory stems from studies carried out in central parts of the distributional range of species, whereas knowledge of how marginal populations function is inadequate. Understanding how marginal populations, living at the edge of their range, function is however in a key position to advance ecology and evolutionary biology as scientific disciplines. My thesis focuses on the factors affecting dynamics of marginal populations of blue mussels (Mytilus edulis) living close to their tolerance limits with regard to salinity. The thesis aims to highlight the dynamics at the edge of the range and contrast these with dynamics in more central parts of the range in order to understand the potential interplay between the central and the marginal part in the focal system. The objectives of the thesis are approached by studies on: (1) factors affecting regional patterns of the species, (2) long-term temporal dynamics of the focal species spaced along a regional salinity gradient, (3) selective predation by increasing populations of roach (Rutilus rutilus) when feeding on their main food item, the blue mussel, (4) the primary and secondary effects of local wave exposure gradients and (5) the role of small-scale habitat heterogeneity as determinants of large-scale pattern. The thesis shows that populations of blue mussels are largely determined by large scale changes in sea water salinity, affecting mainly recruitment success and longevity of local populations. In opposite to the traditional view, the thesis strongly indicate that vertebrate predators strongly affect abundance and size structure of blue mussel populations, and that the role of these predators increases towards the margin where populations are increasingly top-down controlled. The thesis also indicates that the positive role of biogenic habitat modifiers increases towards the marginal areas, where populations of blue mussels are largely recruitment limited. Finally, the thesis shows that local blue mussel populations are strongly dependent on high water turbulence, and therefore, dense populations are constrained to offshore habitats. Finally, the thesis suggests that ongoing sedimentation of rocky shores is detrimental for the species, affecting recruitment success and post-recruit survival, pushing stable mussel beds towards offshore areas. Ongoing large scale changes in the Baltic Sea, especially dilution processes with attendant effects, are predicted to substantially contract the distributional range of the mussel, but also affect more central populations. The thesis shows that in order to understand the functioning of marginal populations, research should (1) strive for multi-scale approaches in order to link ecosystem patterns with ecosystem processes, and (2) challenge the prevailing tenets that origin from research carried out in central areas that may not be valid at the edge.Fluktuationer i populationsstorlek och struktur är ofta reflektioner av förändringar i miljön. En allmänt accepterad lära inom ekologin är den s.k. central-marginal teorin som förutspår att arter generellt förekommer rikligast i centrala delar av deras utbredning, för att minska i antal allt närmare periferin. Detta generella mönster har sitt ursprung i växelverkan mellan arten och dess miljö. Där de biotiska (levande) och abiotiska (icke-levande) processerna är gynnsamma, tenderar individantalet att vara högt medan individantalet minskar mot marginella områden där de fysiologiska och de ekologiska förhållandena närmar sig artens toleransgränser. Förändringar i miljön skapar kontinuerliga förskjutningar i arters utbredning och dessa förändringar är mest markanta vid randen av utbredningen. Eftersom olika processer verkar inom begränsade temporala och rumsliga skalor är det relevant att beskåda fenomenen ur flera perspektiv. Om vi förstår de faktorer som bestämmer dynamiken i randområden, har vi större förmåga att förstå de faktorer som är utslagsgivande i mer centrala populationer. Det har därför globalt förespråkats att studier i randområden är speciellt viktiga och borde erhålla mer vetenskaplig uppmärksamhet. I mina doktorsstudier har jag studerat blåmusslans (Mytilus edulis) populationsdynamik vid randen av dess geografiska utbredning. Finska viken är speciellt intressant med hänsyn till blåmusslan eftersom de abiotiska miljöfaktorerna, huvudsakligen salthalten, förändras dramatiskt inom ett geografiskt litet område. Små abiotiska förändringar kan inom detta område ha drastiska effekter på blåmusslans förekomst och rekryteringsförmåga. Mina studier visar att småskaliga förändringar i salthalt kan ha dramatiska effekter på blåmusslans rekryteringsförmåga. Studierna visar att blåmusslorna på lokal nivå påverkas väsentligt av vågexponering, resulterande i ytterst täta populationer i det yttersta havsbandet för att minska markant mot fastlandet. Resultaten antyder att den ökade sedimenteringen av klippiga bottnar har menliga effekter på musslornas koloniseringsförmåga. Studien förutspår vidare att de ökade mörtbestånden (Rutilus rutilus) i de kustnära miljöerna har ofördelaktiga effekter på blåmusselpopulationerna och att predationstrycket från mört ökar väsentligt från centrala populationer mot randpopulationer. Slutligen antyder studien att biologiska habitatskapare befrämjar blåmusslans koloniseringsförmåga. Avhandlingen bidrar med ny information om Östersjöblåmusslans biologi och motsätter sig tidigare trossatser som antagit att predationstryck och biotisk växelverkan överlag endast har obetydliga effekter på norra Östersjöns blåmusselpopulationer. Trots att blåmusslan är kustekosystemets rikligaste makroskopiska art, och trots att arten innehar en nyckelposition för hela kustekosystemets välbefinnande, har blåmusslans populationsekologi inte studerats i Finland. Detta arbete kan därför ses som en pionjärstudie med såväl teoretisk som praktisk relevans

Explaining Recruitment Stochasticity at a Species' Range Margin

Advancing our understanding of how environmental variability affects the distribution of organisms is crucial for ecology and conservation. The exploration of changes in demographic patterns close to species distribution margins is important as populations here may provide a window into future population changes also elsewhere. However, the knowledge of factors causing recruitment variation is still inadequate in many systems and this deficiency is particularly evident close to species' distribution borders. We studied the spatiotemporal variability in recruit-adult dynamics in a blue mussel, Mytilus trossulus, population to get insights into how environmental variables drive variation in recruitment and how this variability affects adult population growth. Thirty sites along a wave exposure gradient were monitored during four consecutive years. From each site, mussels were collected both from artificial recruitment units and from natural mussel beds. Our results showed high year-to-year variation in recruitment strength with high spatial variation. Mussel recruitment to artificial units and later recruitment to the benthos correlated highly. Juvenile abundances 1 year later paralleled prior recruitment strengths and caused synchronous but time-lagged changes in adult cohorts. Seawater salinity was the strongest predictor for recruitment variation, whereas sea temperature and wave exposure had low predictive power for this early life stage. For juveniles and for adults in the benthos, wave exposure explained the variation best, whereas temperature and especially salinity explained less. The results indicate that (a) the studied blue mussel population is strongly driven by variation in recruitment strength that (b) drives the size of the later cohorts, and the population is possibly even (c) recruitment limited in some years. Our study predicts a challenging future for this range population, resulting from a higher frequency of recruitment failure caused by a deteriorating sea climate. Knowledge about factors underlying variation in recruitment is thus essential for forecasting the future of this range population and for conserving its future state.Peer reviewe

Explaining Recruitment Stochasticity at a Species' Range Margin

Advancing our understanding of how environmental variability affects the distribution of organisms is crucial for ecology and conservation. The exploration of changes in demographic patterns close to species distribution margins is important as populations here may provide a window into future population changes also elsewhere. However, the knowledge of factors causing recruitment variation is still inadequate in many systems and this deficiency is particularly evident close to species' distribution borders. We studied the spatiotemporal variability in recruit-adult dynamics in a blue mussel, Mytilus trossulus, population to get insights into how environmental variables drive variation in recruitment and how this variability affects adult population growth. Thirty sites along a wave exposure gradient were monitored during four consecutive years. From each site, mussels were collected both from artificial recruitment units and from natural mussel beds. Our results showed high year-to-year variation in recruitment strength with high spatial variation. Mussel recruitment to artificial units and later recruitment to the benthos correlated highly. Juvenile abundances 1 year later paralleled prior recruitment strengths and caused synchronous but time-lagged changes in adult cohorts. Seawater salinity was the strongest predictor for recruitment variation, whereas sea temperature and wave exposure had low predictive power for this early life stage. For juveniles and for adults in the benthos, wave exposure explained the variation best, whereas temperature and especially salinity explained less. The results indicate that (a) the studied blue mussel population is strongly driven by variation in recruitment strength that (b) drives the size of the later cohorts, and the population is possibly even (c) recruitment limited in some years. Our study predicts a challenging future for this range population, resulting from a higher frequency of recruitment failure caused by a deteriorating sea climate. Knowledge about factors underlying variation in recruitment is thus essential for forecasting the future of this range population and for conserving its future state

Trophic overlap between expanding and contracting fish predators in a range margin undergoing change

Climate change is predicted to cause a freshening of the Baltic Sea, facilitating range expansions of freshwater species and contractions of marine. Resident marine flounders (Platichthys flesus) and expansive freshwater roach (Rutilus rutilus) are dominant consumers in the Baltic Sea sublittoral where they occur in partial sympatry. By comparing patterns of resource use by flounders and roach along a declining resource gradient of blue mussels (Mytilus trossulus) our aim was to explore predator functional responses and the degree of trophic overlap. Understanding the nature of density-dependent prey acquisition has important implications for predicting population dynamics of both predators and their shared prey. Results showed a highly specialized diet for both species, high reliance on blue mussels throughout the range, similar prey size preference and high trophic overlap. Highest overlap occurred where blue mussels were abundant but overlap was also high where they were scarce. Our results highlight the importance of a single food item - the blue mussel - for both species, likely promoting high population size and range expansion of roach. Findings also suggest that range expansion of roach may have a top-down structuring force on mussels that differ in severity and location from that originating from resident flounders.Peer reviewe

Lagoon morphology as an overarching driver for perch breeding success

Fish spawning and nursery habitats are critically important for the maintenance and conservation of viable fish stocks. Understanding habitat use of breeding fish is therefore important. This study examines the spatial distribution of Eurasian perch (Perca fluviatilis) egg strand occurrence and larval density in 18 brackish water post-glacial land-uplift lagoons in the northern Baltic Sea. The aim of the study was to quantify spawning habitat characteristics and evaluate how geomorphological, hydrological, and biological parameters affect perch breeding. Egg strand occurrence was assessed by snorkeling whereas fish larvae and zooplankton were sampled using horizontal surface hauls. Egg strand and larval density increased with higher habitat isolation, i.e., bay morphology had a decisive role in determining early recruitment success. Moreover, egg strand occurrence and larval density correlated positively with water temperature, charophyte cover and negatively with salinity. The zooplankton community structure differed among lagoon types but neither zooplankton density nor diversity showed a clear association with egg strand or fish larvae abundance. However, cladocerans and copepods were abundant in the most enclosed bays, and their density correlated positively with perch larval density. Our findings comply with earlier studies highlighting the importance of bay isolation for fish recruitment strength. We call for a foreseeing coastal management planning, that apart from considering current status and trends, also integrates decadal long projections of isostatic land-uplift processes in conservation decisions to secure the long-term persistence of the best spawning and nursery areas

Template for using biological trait groupings when exploring large-scale variation in seafloor multifunctionality

Understanding large-scale spatial variation in ecosystem properties and associated functionality is key for successful conservation of ecosystems. This study provides a template for how to estimate differences in ecosystem functionality over large spatial scales by using groupings of biological traits. We focus on trait groupings that describe three important benthic ecosystem properties, namely bioturbation, community stability, and juvenile dispersal. Recognizing that groups of traits interact and are constrained within an organism, we statistically define important functional trait subgroups that describe each ecosystem property. The sub-groups are scored according to their weighted ecological impact to gain an overall estimation of the cumulative expression of each ecosystem property at individual sites. Furthermore, by assigning each property a value relative to its observed maximum, and by summing up the individual property values, we offer an estimate of benthic ecosystem multifunctionality. Based on a spatially extensive benthic data set, we were able to identify coastal areas with high and low potential for the considered benthic ecosystem properties and the measure of ecosystem multifunctionality. Importantly, we show that a large part of the spatial variation in functional trait sub-groups and in benthic ecosystem multifunctionality was explained by environmental change. Our results indicate that through this simplification it is possible to estimate the functionality of the seafloor. Such information is vital in marine spatial planning efforts striving to balance the utilization with the preservation of natural resources.Peer reviewe

Fladojen ja kluuvien kunnostus kalojen lisääntymisalueiksi : Kokemuksia kunnostuksista ja tuloksellisuuden mittausmenetelmistä

Fladojen ja kluuvien tiedetään olevan tärkeitä lisääntymisalueita erityisesti ahvenelle, hauelle ja särkikaloille. Kiinnostus niiden kalataloudellisiin kunnostuksiin on kasvamassa. Euroopan meri- ja kalatalousrahaston (EMKR) rahoittamassa kalatalouden ympäristöohjelmassa tehtiin vuosina 2019–2022 yhdeksän kunnostuskokeilua Merenkurkun ja Suomenlahden fladoissa ja kluuveissa. Yksinkertaista kvalitatiivista tietoa siitä, käyttävätkö kalat kohdetta lisääntymisalueena, voidaan kohtalaisen helposti saada jo muutaman hyvin ajoitetun käynnin perusteella. Tarkempaa määrällistä tietoa kohteen merkityksestä ahvenen lisääntymisalueena saadaan esimerkiksi laskemalla mätinauhoja joko pinnan yläpuolelta tai snorklaamalla. Dronekuvien avulla tehty mätinauhojen laskenta osoittautui hankalaksi ja epävarmaksi. Vastakuoriutuneiden ahvenpoikasten tiheyden arviointi vetohaavipyydyksellä antoi samansuuntaisia tuloksia eri kohteista kuin mätilaskennat. Kluuveihin ja myös fladoihin keväällä kutemaan nousevien kalojen määriä voidaan arvioida riistakameralla. Menetelmä on melko työläs, erityisesti kuvamateriaalin läpikäymisen osalta, ja eri lajien erottaminen kuvista oli ajoittain vaikeaa. Riistakameralla saatujen tulosten yhdistäminen rysällä tehdyn näytteenoton tuloksiin voi antaa hyvän kuvan kohteen kutupopulaatioista. Kolmessa läntisellä Suomenlahdella sijaitsevassa kohteessa tavoitteena oli palauttaa ahven kutemaan fladaan tai kluuviin, joissa olemassa olevien lähtötietojen perusteella ahven ei aiemmin enää kutenut. Näistä kohteista kahdessa saatiin melko rohkaisevia tuloksia, sillä kevyiden käsivoimin tehtyjen kunnostustoimien jälkeen ahvenia kuti kohteissa runsaasti. Kolmannessa kohteessa ahventen saaminen kutemaan selkeästi epäonnistui. Kahdessa Suomenlahden ja kolmessa Merenkurkun kluuvikohteessa tavoitteena oli turvata heikentymässä olleita kalojen kulkuyhteyksiä tuleviksi vuosiksi tai vuosikymmeniksi. Kunnostukset toteutettiin pääosin koneellisesti. Kohteista kerättyjen aineistojen perusteella toimenpiteillä ei ollut havaittavia lyhytaikaisia haittavaikutuksia kohteiden vedenlaatuun, poikastuotantoon tai kutukantaan. Selkämerellä Kristiinankaupungin alueella tehdyssä kunnostuskokeilussa muokattiin koko kluuvia melko voimaperäisesti kaivinkoneilla. Siellä hauen pienpoikamäärät moninkertaistuivat kunnostuksen jälkeisinä vuosina. Kunnostuskokeiluissa tavoiteltujen vaikutusten pysyvyydestä saadaan kuitenkin luotettavaa tietoa vasta vuosien kuluttua. Flada- ja kluuvikunnostuksia koskevan tiedon ja kokemusten keruussa on vasta päästy alkuun. Jatkossa tulisi entistä systemaattisemmin ja monipuolisemmin kerätä tietoa sekä veden laadusta että kalastosta useiden vuosien ajan ennen kunnostuksia ja kunnostusten jälkeen

Wave stress and biotic facilitation drive community composition in a marginal hard-bottom ecosystem

Abstract Ecological patterns are inherently scale-dependent and driven by the interplay of abiotic gradients and biotic processes. Despite the fundamental importance of such gradients, there are many gaps in our understanding of how abiotic stress gradients interplay with biotic processes and how these collectively affect species distributions. Using a hierarchical design, we sampled two communities separated by depth along wave exposure and salinity gradients to elucidate how these two gradients affect species composition in habitats formed by the foundation species Mytilus trossulus and Fucus vesiculosus. Specifically, we looked at the impacts of regional salinity and temperature, local wave exposure, and site-dependent facilitation effects on the associated community composition. Wave exposure was the best predictor for species assembly structure, which was also affected by Mytilus biomass and by salinity and water temperature. While the tested variables provided robust explanations for community structure and density, they did not provide conclusive explanations for variation in species richness or evenness. Mytilus biomass had a stronger effect on the associated community with increasing wave exposure at the deeper depth, but the patterns were less obvious at the shallower depth. The latter was also the case for Fucus. These findings comply partly with theoretical predictions suggesting stronger facilitation effects in physically harsh environments. Our results indicate that environmental drivers are the main structuring forces that affect species assembly structure, but also foundation species are important. Thus, predicting changes in species distributions and biodiversity requires the simultaneous consideration of environmental gradients, as well as the structure and composition of foundation species and the interplay between these factors. This work advances our understanding of the processes that modulate species distributions in a marginal marine area and broadens the knowledge of how biological and environmental factors interplay and have an influence on hard-bottom community structure in brackish water seas.peerReviewe