Hartvig Christie

North Atlantic phylogeography and large-scale population differentiation of the seagrass Zostera marina L.

As the most widespread seagrass in temperate waters of the Northern Hemisphere, Zostera marina provides a unique opportunity to investigate the extent to which the historical legacy of the last glacial maximum (LGM18 000–10 000 years bp) is detectable in modern population genetic structure. We used sequences from the nuclear rDNA–internal transcribed spacer (ITS) and chloroplast matK‐intron, and nine microsatellite loci to survey 49 populations (> 2000 individuals) from throughout the species’ range. Minimal sequence variation between Pacific and Atlantic populations combined with biogeographical groupings derived from the microsatellite data, suggest that the trans‐Arctic connection is currently open. The east Pacific and west Atlantic are more connected than either is to the east Atlantic. Allelic richness was almost two‐fold higher in the Pacific. Populations from putative Atlantic refugia now represent the southern edges of the distribution and are not genetically diverse. Unexpectedly, the highest allelic diversity was observed in the North Sea–Wadden Sea–southwest Baltic region. Except for the Mediterranean and Black Seas, significant isolation‐by‐distance was found from ~150 to 5000 km. A transition from weak to strong isolation‐by‐distance occurred at ~150 km among northern European populations suggesting this scale as the natural limit for dispersal within the metapopulation. Links between historical and contemporary processes are discussed in terms of the projected effects of climate change on coastal marine plants. The identification of a high genetic diversity hotspot in Northern Europe provides a basis for restoration decisions.

EFFECTS OF DENSITY-DEPENDENT AND STOCHASTIC PROCESSES ON THE REGULATION OF COD POPULATIONS

We analyzed 136 time series (covering from 44 to 73 yr) of juvenile cod to estimate the level of direct and delayed density-dependent mortality (DDM) of 11 popu- lations from the Norwegian Skagerrak coast. The parameters were estimated using a mod- eling approach that explicitly incorporates observation errors, so that we could quantify the density-independent (stochastic) variation in the survival of juvenile cod. Moderate to strong levels of DDM (direct or delayed) were estimated in eight of the 11 populations. Variability in the 0-group (corrected for observation errors) appeared to be large for most of the populations. Substantial stochastic variability in postsettlement survival was also detected in some areas, indicating that stochastic factors are not only important for egg and larval stages, as stated by the match-mismatch hypothesis, but also for juveniles. We show that the variability in these coastal populations is not only regulated as a function of the strength of DDM processes, but also as an interaction between DDM processes and stochastic factors. We finally postulate that local and regional differences in the strengths of the density-dependent and stochastic processes are related to differences in the quantity and quality of the bottom flora coverage, which govern both food availability and shelter for juveniles.

Fish-macrofauna interactions in a kelp (Laminaria hyperborea) forest

predator–prey interactions between fish and macrofauna in kelp ( laminaria hyperboea ) forest were examined. fish were caught in gill-nets and fish traps at two sites and throughout the diurnal cycle. their stomach contents were analysed. the most abundant fish species, except pollachius virens , were caught in larger numbers at night than during daytime. the examined fish fed throughout the diurnal cycle and kelp-associated fauna were found to be important prey. pollachius virens was caught in the same numbers throughout the diurnal cycle and contained mainly pelagic prey, in larger numbers during the day than at night. small labrids were specialists, feeding mainly on the two numerically dominant kelp-associated gastropods rissoa parva and lacuna vincta . the larger labrus bergylta , gadus morhua and myoxocephalus scorpius were generalists, feeding on a large range of kelp-associated invertebrates. some fish species showed preference for particular vertical levels when feeding in the kelp forest. the most mobile and/or the largest invertebrates were most heavily preyed upon.

Effects of removing sea urchins (Strongylocentrotus droebachiensis): Stability of the barren state and succession of kelp forest recovery in the east Atlantic

Stability properties of the barren state of a kelp forest-sea urchin system were studied in northern Norway. The ability of the sea urchin Strongylocentrotus droebachiensis to maintain high population densities and recover from perturbations, and the succession of kelp forest revegetation, were studied experimentally by reducing the sea urchin density on a barren skerry. Additional information was obtained from community changes following a natural, but patchy, sea urchin mortality that varied between sites. On the barren grounds, high sea urchin densities (30 50 per m2) is maintained by annual recruitment. Severe reductions of sea urchin densities initiated luxuriant kelp growth, while more moderate reductions allowed establishment of opportunistic algae (during spring and early summer), but no kelps. Succession of algal growth, after the severe decline in sea urchin densities, followed a predictable pattern. At first the substrate was colonized by filamentous algae, but within few weeks they were outcompeted by the fast growing kelp Laminaria saccharina. After 3–4 years of the removal experiment, the slower-growing, long-lived kelp L. hyperborea became increasingly dominant. Increased food availability after reduction in sea urchin density led to increased individual growth of the remaining sea urchins. However, the population density did not increase, neither from recruitment nor immigration from adjacent areas with high sea urchin densities. Possibly, early establishment of a dense kelp stand, may represent a breakpoint in the ability of sea urchins to reestablish a barren state. The ability of L. saccharina quickly to invade and monopolize an area may have both positive and negative effects on the succession towards the climax L. hyperborea kelp forest. Competitive interactions may slow the process, but development of a dense stand of L. saccharina will also reduce grazing risk on scattered recruits of the more slowly growing L. hyperborea.

Large-scale shift from sugar kelp (Saccharina latissima) to ephemeral algae along the south and west coast of Norway

Abstract Sugar kelp forests (Saccharina latissima) normally dominate the sublittoral rocky sea bed in medium exposed to sheltered areas of the Norwegian coast. In 2002, a large-scale disappearance of sugar kelp was observed, and a survey of more than 600 sites along the coast of southern Norway (58–63°N, 6–11°E) during 2004–2009 recorded a large-scale shift from anticipated sugar kelp forests to communities dominated by filamentous, ephemeral macroalgae. The loss of sugar kelp was most severe in the Skagerrak region, and a comparison with earlier studies supported the observed changes in the community structure and regional differences between the Skagerrak region and the west coast. Filamentous red algae dominated the sublittoral communities at the Skagerrak coast, whereas coarsely branched and filamentous brown algae were most abundant at the west coast sites with low or no abundance of sugar kelp. The cause of this large-scale shift is not clear, but our observations suggest eutrophication (nutrient and particle pollution) and climate change (increase in temperature) as the two main drivers synergistically contributing, alongside other factors, to the demise of S. latissima. If the ephemeral algae community represents an alternative stable state, it will have implications for marine coastal zone production and management.

Sea urchin grazing and kelp re-vegetation in the NE Atlantic

Abstract Small-scale grazing events where sea urchins have grazed kelp forests to barren grounds have been reported all along the NE Atlantic coast. One large-scale event has been reported where kelp forests along the Norwegian and Russian coast were grazed by sea urchins during the early 1970s. The barren ground area has persisted since. Different theories to explain the grazing event have been presented. This paper seeks to sort and summarize earlier published papers and national reports and to critically examine the most important theories presented to explain the grazing event. The conclusion is that the reason for the event is unknown and it is too late to find causes 40 years after it took place. However, new data and new geographical analysis tools provide insight into the extent and consequences of this dramatic event, and show re-vegetation of kelp forests in the southernmost area. Emphasis in future studies should be given to understand the reasons for shifts between the two ecosystem states and take advantage of the ongoing shift. Such basic ecological knowledge could provide an important basis for better understanding the system and further, to understand the extent to which other observed large-scale changes (e.g. climatic changes, fish stock reductions) affect kelp forest–sea urchin interaction.

The Response of Experimental Rocky Shore Communities to Nutrient Additions

The aim of this study was to determine whether the experimental nutrient enrichment of littoral rocky shore communities would be followed by a predicted accumulation of fast-growing opportunistic algae and a subsequent loss of perennial benthic vegetation. Inorganic nitrogen (N) and potassium (P) was added to eight concrete mesocosms inhabited by established littoral communities dominated by fucoids. The response to nutrient enrichment was followed for almost 2 1/2 years. Fast-growing opportunistic algae (periphyton and ephemeral green algae) grew significantly faster in response to nutrient enrichment, but the growth of red filamentous algae and large perennial brown algae was unaffected. However, these changes were not followed by comparable changes in the biomass and composition of the macroalgae. The biomass of opportunistic algae was stimulated only marginally by the nutrient enrichment, and perennial brown algae (fucoids) remained dominant in the mesocosm regardless of nutrient treatment level. Established rocky shore communities thus seem able to resist the effects of heavy nutrient loading. We found that the combined effects of the heavy competition for space and light imposed by canopy-forming algae, preferential grazing on opportunistic algae by herbivores, and physical disturbance, succeeded by a marked export of detached opportunistic algae, prevented the fast-growing algae from becoming dominant. However, recruitment studies showed that the opportunistic algae would become dominant when free space was available under conditions of high nutrient loading and low grazing pressure. These results show that established communities of perennial algae and associated fauna in rocky shore environments can prevent or delay the accumulation of bloom-forming opportunistic algae and that the replacement of long-lived macroalgae by opportunistic species at high nutrient loading may be a slow process. Nutrient enrichment may not, in itself, be enough to stimulate structural changes in rocky shore communities.

Regrowth of kelp and colonization of epiphyte and fauna community after kelp trawling at the coast of Norway

The kelp Laminaria hyperborea is regularly harvested along the Norwegian coast. Kelp trawling is regulated by restricting this to every 5th year in specified areas. The kelp plants form dense forests, 1–2 m high, and house a large number of epiphytes and associated invertebrates. Kelp, epiphytes, and holdfast (hapteron) fauna were sampled at two different regions in untrawled kelp forest and at sites trawled different number of years ago. We have examined the rate of kelp regrowth after trawling, and in what time scale the associated flora and fauna colonize the trawled areas. The trawl removed all adult kelp plants (the canopy plants), while small understorey kelp plants were left undisturbed. These recruits, given improved light conditions, made the new generation of canopy-forming kelp plants, exceeding a height of 1 m within 2–3 y. The recruitment pattern of the kelp ensures maintenance of kelp forest dominance despite repeated trawling. Both percent cover, abundance and number of epiphytic species increased with time post trawling, but epiphytic communities were not totally re-established before the next trawling episode. Colonization of most species of fauna inhabiting the kelp holdfast were found as early as one year after trawling, but increasing size of the habitat by age of kelp gave room for increasing numbers of both individuals and species. Slow colonization rate by some species might be due to low dispersal potential. Due to a higher maximum age and size of kelp plants in the northernmost region studied, restoration of both kelp and kelp forest community was slower there.

Species richness in macroalgae and macrofauna assemblages on Fucus serratus L. (Phaeophyceae) and Zostera marina L. (Angiospermae) in Skagerrak, Norway

The macrophytes Fucus serratus and Zostera marina form similar substrates for associated flora and fauna in shallow waters in Norway. While F. serratus forms a more or less continuous belt on rocky substrate along the coast, Z. marina forms disjunct populations on sandy or muddy bottoms. This study focused on the organisms associated with these two macrophytes in two localities in the Skagerrak region. In total, 130 taxa of epiphytic organisms were identified: 22 green algae, 41 red algae, 32 brown algae, eight diatoms and 27 sessile animals. One hundred and twenty-seven taxa of mobile macrofauna were registered. The dominant group was crustaceans, with amphipods as the order containing most species. Many species of both plants and animals preferred one or the other habitat. It is concluded that coastal macrophyte systems have high species diversity.

Infauna fromZostera marina L. meadows in Norway. Differences in vegetated and unvegetated areas

Abstract To investigate the importance of benthic vegetation for shallow infaunal assemblages, macroinvertebrates were sampled inZostera marina meadows and in adjacent bare sand at three sites in the Skagerrak region and one in the Western part of Norway. A total of 3777 individuals were encountered, representing mainly the faunal groups Annelida, Nemertea, Echinodermata, Crustacea and Mollusca. Twice as many individuals were found in theZ. marina meadows compared to the bare sand. Oligochaetes were the most abundant group (29% of total number of individuals), followed by crustaceans (24%), polychaetes (18.5%) and molluscs (16%). Polychaetes showed the highest species number (42), followed by molluscs (30) and crustaceans (26). Multivariate cluster technique showed the highest similarities within sites and thereafter grouping according to the two different habitats;Zostera and sand. No regional differences were evident between the Skagerrak sites and the site in Western Norway, as the site from Western Norway grouped together with one of the Skagerrak sites. The high biodiversity inZostera marina beds should have implications for the management of such areas.