Solution:The correct sequence of ecosystems in the order of decreasing productivity is - Mangroves, grasslands, lakes, oceans.Mangroves: These coastal ecosystems are highly productive due to their location, abundant sunlight, and nutrient-rich environments.
Grasslands: Grasslands also exhibit high productivity, supporting a diverse range of herbivores.
Lakes: Lakes have moderate productivity, relying on external sources of nutrients and sunlight.
Oceans: Oceans, particularly the open ocean, have the lowest productivity due to their vast size, nutrient-poor waters, and limited sunlight penetration.