Grazing pressure

The recent increase in the understanding of biological processes in lakes has led to the development of ecotechnical methods of manipulating the trophic status of lakes. The most widely used techniques of bio-manipulation involve artificial change in the abundance of predators to enhance grazing of phytoplankton by zooplankton. The increase in grazing pressure reduces phytoplankton densities and results in improved water transparency. 

Owing to the costs of defenses, natural selection will optimize defenses. There are various aspects in this optimization, including defense allocation with respect to temporal variation in grazing pressure and within-plant defense allocation with respect to grazing risk and the value of different plant parts (reviewed in Stamp 2003). Here we will focus on both the induced defenses and within-plant allocation of defenses, because these aspects have been widely studied in macroalgae. 

VHOCs was also observed in response to grazing pressure in Ascophyllum nodosum beds (Nightingale et al. 1995). Their biogenesis involves vanadium haloperoxi-dases (vHPO), which catalyse the oxidation of halides (X ) and generate X+ to yield hypohalous acid (XIO) and other forms of oxidized halides (X2, X2 ). Marine organisms and especially seaweeds have been known for a long time to concentrate halides from their environment (for a recent review, see Leblanc et al. 2006). 

The prevailing view of the dynamics of many phytoplankton blooms is that they often terminate due to nutrient exhaustion, increased grazing pressure, and/or physical dispersal. Evidence is now accumulating that tamarensis blooms have ended when grazing and advection were low and nutrients were above detection limits (18, 21). Instead of persisting without division as nutrients disappear... 

A final complicating factor is that concentrations of these chemicals can be affected by the organism s environment. Secondary metabolite concentrations in marine macroalgae can be influenced by or correlated with light intensity,49 particularly in the UV range,70 grazing pressures,43"16 nutrient concentrations,44 45 49 73 74 desiccation,75 and salinity.76... 

Hansen FC, van Boekel WHM (1991) Grazing pressure of the calanoid copepod Temora longicornis on a Phaeocystis dominated spring bloom in a Dutch tidal inlet. Mar Ecol Prog Ser 78 123-129... 

---