Q.State the two kinds of units in which the primary productivity of an ecosystem is usually expressed.

Productivity is the rate at which an ecosystem manufactures new biomass per unit area per unit time, expressed in g/m²/year or Kcal/m²/year. Before any productivity can occur, a producer must first capture usable sunlight — only the narrow 400–700 nm waveband, called Photosynthetically Active Radiation (PAR), is actually usable for photosynthesis, and PAR itself fluctuates with cloud cover, shade, dust, season, latitude and day length. Of all the sunlight reaching the atmosphere, about 34% is reflected away, 10% is absorbed by ozone/water vapour/gases, and only the remaining 56% reaches the surface — of which green plants capture just 2–10% for photosynthesis, the rest being lost as heat. Productivity itself is classed three ways. Primary productivity is the organic matter/chemical energy autotrophs generate via photosynthesis and chemosynthesis; Gross Primary Productivity (GPP) is the total amount produced, while Net Primary Productivity (NPP) is what remains after the plant's own respiratory losses (NPP = GPP − Respiration) — global NPP is estimated at ~170 billion dry-weight tonnes/year, of which oceanic producers contribute ~55 billion tonnes. Secondary productivity is the energy stored in consumer tissue, split into gross (total plant material ingested minus faecal loss) and net (what is …