what are contrasts between a marine and estuarine environment?

Tides do not influence water levels as much in an estuary environment. The water is brackish and varies in salinity there.

Reading this will help you:
http://fds.oup.com/www.oup.com/pdf/13/9780198525080.pdf

Can I also mention in my answer that an estuarine environment is part of a marine environment?

I do not recommend doing that. One is not a subclass of the other.

To understand the contrasts between a marine and estuarine environment, let's first define these two terms:

1. Marine Environment: A marine environment refers to the saltwater ecosystem found in oceans or seas. It includes the open ocean as well as marine habitats along coastlines.

2. Estuarine Environment: An estuarine environment is a partially enclosed body of water where freshwater from rivers and streams meets and mixes with saltwater from the ocean. Estuaries can be found where rivers flow into the ocean, creating a dynamic transition zone.

Now, let's explore the contrasts between these two environments:

1. Salinity: In a marine environment, the salinity levels are relatively high due to the presence of seawater. In contrast, estuarine environments have varying salinity levels as freshwater and saltwater mix, leading to brackish conditions. The salinity in estuaries fluctuates with tidal movements, changing as freshwater flows in or seawater intrudes.

2. Biodiversity: Both marine and estuarine environments are rich in biodiversity, but the species composition differs. Marine environments harbor a wide range of marine species, including pelagic fish, coral reefs, and marine mammals. Estuarine environments, on the other hand, support a unique mix of freshwater and marine species. Many estuaries serve as important nurseries for various fish species while also providing habitats for migratory birds and other aquatic organisms.

3. Physical Characteristics: Marine environments tend to be vast, with deeper depths, strong currents, and extensive wave action. In contrast, estuarine environments are typically smaller and shallower, with slower-moving water and less wave energy. Estuaries often have muddy sediments due to the influx of freshwater and sediment from rivers.

4. Nutrient Availability: Estuarine environments receive a significant amount of nutrients from freshwater runoff, including sediments, organic matter, and nutrients like nitrogen and phosphorus. This high nutrient input supports abundant primary productivity and provides a valuable source of food for various organisms. In marine environments, while there is still some nutrient input from rivers and ocean currents, it is generally lower than in estuaries.

5. Human Impact: Both marine and estuarine environments face human impact, but the nature of these impacts can vary. Marine environments are subjected to threats like overfishing, habitat destruction (e.g., coral bleaching), pollution, and sea level rise. Estuarine environments also encounter these threats but, due to their proximity to human settlements, may face additional challenges related to urban development, agriculture, and industrial activities.

Understanding the contrasts between marine and estuarine environments helps us appreciate the unique ecological characteristics of each and highlights the importance of conservation and management efforts for their sustainability.