The CO2 balance of unproductive aquatic ecosystems

Community respiration (R) rates are scaled as the two-thirds power of the gross primary production (P) rates of aquatic ecosystems, indicating that the role of aquatic biota as carbon dioxide sources or sinks depends on its productivity. Unproductive aquatic ecosystems support a disproportionately higher respiration rate than that of productive aquatic ecosystems, tend to be heterotrophic (R > P), and act as carbon dioxide sources. The average P required for aquatic ecosystems to become autotrophic (P > R) is over an order of magnitude greater for marshes than for the open sea. Although four-fifths of the upper ocean is expected to be net heterotrophic, this carbon demand can be balanced by the excess production over the remaining one-fifth of the ocean.     

OBSERVATIONS ON THE EPIDEMIOLOGY OF PORCINE PARVOVIRUS

Evidence presented suggests that porcine parvovirus is highly stable and infective. Introduction of virus to susceptible herds results in 100% infection rate within the following 3 months. Active immunity is associated with high persistent levels of haemagglutination-inhibitating (HI) antibody (> 256), piglets suckling immune sows acquiring HI titres between 10,000 and 40,000. Loss of passive immunity, measured by HI, occurs in a majority of pigs between 14 and 26 weeks of age (mean 21 weeks), whilst an average of 25% (2–47%) of pigs lose HI titres between 26 and 36 weeks of age. Susceptibility to challenge with virus does not occur until 3–5 weeks following loss of HI titres. In endemically infected herds 98–100% of adult pigs show serological evidence of active immunity.
A significant proportion of gilts may not be actively immune to porcine parvovirus at the time of first service, and subsequent infection may occur while these gilts are pregnant.