Which one of following exhibits least productivity?

In oceans, there are lots of nutrients but no sufficient sunlight is available in deep areas hence, oceans have the least productivity. Grasslands are less productive ecosystem with annual productivity of 200?1000 gm/m2. Maximum productivity can be seen in coral reefs followed by estuaries. Salt marshes are estuaries where brackish and salt water mix. Salt marsh plants(halophytes) are salt tolerant and adapted to water levels that fluctuate with tides. Tides carry in the nutrients that stimulate plant growth in marshes.