Top-down and Bottom-up controls on a range of r- and K-selected macroalgae in a Danish estuary: Top-down og Bottom-up kontroller på en række af r- og K-selekterede makroalger i et dansk estuarie

Rasmus Ekberg Villumsen

Studenteropgave: Speciale

Abstrakt

Nutrient enrichment of shallow coastal ecosystems changes the composition of plant communities from slow growing K-selected benthic macrophytes, to fast growing r-selected algae, such as ephemeral algae. Enhancing this effect is the decline in big predatory fish, which by cascading trophic effects relives algae from grazing control, favoring the r-selected species which are more susceptible to herbivorism. These scenarios suggest that small r-selected species will suffer more from nutrient limitation and grazing than slower growing K-selected species at low nutrient availability and high grazing pressure. This hypothesis was tested, by comparing the effect of in situ nutrient enrichment and presence of invertebrate grazers on 3 ephemeral macroalgae (Ulva lactuca, Cladophora sericea, and Ceramium rubrum) and 3 perennial macroalgae (Gracilaria vermiculophylla, Fucus serratus, and Fucus vesiculosus). Concentrations of DIN and DIP in the water indicated that algae were N-limited in September and then P-limited in the early spring, and then again becoming N-limited during summer. During winter where both DIN and DIP concentrations were relative high, the algae were limited by abiotic factors (temperature and light). Tissue N and P concentration further indicated that especially U. lactuca, but also G. vermiculophylla and C. rubrum suffered from both N and P in early summer. The bigger Fucus spp. did experience a decrease in tissue concentration of both N and P through spring and summer, but the internal reserves gathered through winter were much greater, and they were therefore not nutrient limited to the same extent as the other algae. Bottom-up forces had the most impact on the growth rates of the algae in the experiments, as 2 of the algae had an overall positive effect from nutrient enrichment. Top-down forces had an overall positive effect on the K-selected G. vermiculophylla, whilst having a significant effect on r-selected C. rubrum in the individual experiment, reducing its relative growth rate with 143 % in June. The biggest effect of nutrient enrichment was on the very r-selected U. lactuca, which increased its relative growth rate by 116 % due to nutrient enrichment in June. The results indicate that bottom-up forces mainly control the biomass of the algae in the Isefjord ecosystem, but that top-down forces can be equally and more effective than bottom-up forces, and that the bottom-up and top-down forces mainly impact r-selected species.

UddannelserMiljørisiko, (Bachelor/kandidatuddannelse) Kandidat
SprogEngelsk
Udgivelsesdato12 jul. 2017
Antal sider36
VejledereMorten Foldager Pedersen

Emneord

  • Top-down
  • Bottom-up
  • Eutrophication
  • Macroalgae
  • r-selection
  • K-selection
  • Estuary
  • softbottom
  • grazing
  • growth rates
  • Nitrogen
  • Phosphorus
  • Ulva lactuca
  • Gracilaria vermiculophylla
  • Fucus serratus
  • Fucus vesiculosus
  • Ceramium rubrum
  • Cladophora sericea