Geographers analyzed 175 satellite images of ocean colour, which is an indicator of Phytoplankton productivity at the ocean’s surface, and found that giant icebergs are responsible for storing up to 20 percent of carbon in the Southern Ocean. The researchers discovered that melting water from giant icebergs which contains iron and other nutrients, supports hitherto unexpectedly high levels of phytoplankton growth.

Explanation:

Assumption 1: Giant icebergs have a bearing on primary productivity and food chains of the Southern Ocean.

This assumption appears to be valid. The passage states that:
– Giant icebergs are responsible for storing up to 20 percent of carbon in the Southern Ocean.
– Melting water from giant icebergs contains iron and other nutrients.
– These nutrients support unexpectedly high levels of phytoplankton growth.

Phytoplankton are at the base of the marine food chain and are responsible for primary productivity in the ocean. If giant icebergs significantly affect phytoplankton growth, they would indeed have a bearing on primary productivity and, by extension, the food chains of the Southern Ocean.

Assumption 2: Melting of giant icebergs can produce climate change effects and impact world fisheries.

While this assumption seems logical, it’s not directly supported by the information given in the passage. The passage doesn’t mention climate change effects or impacts on world fisheries. It only discusses the role of icebergs in carbon storage and phytoplankton growth in the Southern Ocean.

Therefore, based solely on the information provided in the passage:

Assumption 1 is valid.
Assumption 2, while plausible, is not directly supported by the given information.

The correct answer is a) 1 only.