The correct sequence of ecosystems in the order of decreasing productivity—mangroves, grasslands, lakes, and oceans—reflects the varying capacities of these ecosystems to produce biomass through photosynthesis. Mangroves are highly productive due to their nutrient-rich environments and ability to support diverse flora and fauna, while grasslands, though productive, have limited biomass compared to the specialized conditions of mangroves. Lakes can support significant productivity, but they are generally lower in comparison to the extensive biomass in both mangrove and grassland ecosystems, and while oceans have vast surface areas, their overall productivity is often limited due to nutrient distribution and depth. This information is crucial for understanding ecological dynamics and resource management, pertinent to the UPSC syllabus on biodiversity and ecosystem studies.