lecture 5 Eutrophication and Sedimentation: Bottom-up Destruction of Marine Ecosystems

Eutrophication is a biological process initiated by excessive nutrient enrichment, leading to increased primary production.
It involves:
- Increased primary production: Transition from macrophyte productivity (larger plants) to increased phytoplankton production.
- Formation of sporadic toxic blooms: Especially from dinoflagellates, leading to phenomena such as red tides.
- Hypoxia: Low oxygen levels due to bacterial respiration when organic materials decompose.
- Anoxia: Complete lack of oxygen that results in “dead zones”.

Radical changes in marine ecosystems can arise from both top-down and bottom-up processes.
Pycnocline: A layer in the ocean where there is a rapid change in water density with depth, often due to temperature or salinity gradients.
Dead zones: Areas in aquatic environments where oxygen levels are too low to support marine life.
Wasting disease: A disease affecting seagrasses, leading to extensive losses in their populations.

Global distribution: Marine environments have their highest primary productivity around coasts and in enclosed seas, heavily influenced by nutrient supply from terrestrial sources and oceanic upwellings.
Notably, about 4 billion people live less than 400 km from the coast, and more than 80% of coastal habitats have been modified by human activities.
Marine productivity vs. Terrestrial productivity:
- Marine habitats account for approximately 40% of total global production.
- Roughly 40% of terrestrial productivity has been impacted by human activity compared to around 5% in oceans.

Depth and light availability significantly affect marine productivity.
- Nutrient concentration and light intensity dictate productivity across various ocean depths.
Temperature and mixing processes, such as thermoclines, play crucial roles in regulating nutrient distribution.

Kelp forests found in temperate rocky substrates exhibit high productivity.
Coral reefs in tropical areas show high levels of macroalgae and turf growth.
Seagrass beds thrive in temperate and tropical sandy substrates and are crucial for several marine species.

Toxic dinoflagellate example: Gonyaulax species can cause red tides, resulting in detrimental effects on marine life and habitat.
- Areas such as South Australia have reported threats to specific marine species following blooms.

Nutrients, mainly from agricultural runoff and urban sources, are delivered during storm events, contributing to nutrient enrichment.
Effects of nutrient influx:
- Increased surface production leads to organic matter sinking and decomposition in deeper waters.
- The pycnocline inhibits oxygen resupply to deeper layers, leading to mortality among less mobile organisms.

Case studies; Data trends showing:
- Increased nitrates in water bodies like the Black Sea and Baltic Sea from 1970-1990.
- Decreased dissolved oxygen levels alongside increased red tides in coastal areas like Chesapeake Bay.
Fishery impacts: Documented declines in catches of species like cod and lobster in the Baltic Sea due to hypoxia.

Dead zone expansion:
- In 1960, 49 sites existed; by 2008, this escalated to 400 sites covering approximately 245,000 km².
- The largest dead zone in the Baltic Sea now lacks oxygen year-round.
- Significant increases continued post-2008, with over 500 sites reported by 2018.

Hypoxia arises from nutrient over-enrichment, exacerbated by higher ocean temperatures.
The Gulf of Mexico dead zone, linked to runoff from the Mississippi River, exemplifies the effects of agricultural practices on marine environments.
Cycle of formation: Seasonal patterns wherein fresh water runoff leads to barriers preventing oxygen resupply to deeper saline waters, subsequently leading to algae blooms and their decay—a process consuming available oxygen.

Seagrass is a critical subtidal vegetated habitat and hosts around 12 genera and ~60 species, covering approximately 600,000 km² globally (0.5% of oceanic area).
Main functions include:
1. High productivity ranging between 500-4000 g C/m²/year.
2. Nutrient cycling (Nitrogen, Phosphorus, Carbon, Sulfur).
3. Providing food resources for diverse marine species.
4. Stabilizing sediments.
5. Offering shelter, nursery, and breeding grounds for marine life.

Coral reefs face bottom-up stress from nutrient enrichment, sedimentation, and increased algal cover, resulting in decreased coral cover and growth rates.
Kaneohe Bay case study: Examples of how recovery can occur with reduced nutrient inputs from sewage diverting measures in the late 1970s leading to noted improvements in coral health.

Seagrass habitats suffer tremendous losses—up to 30% (or ~200,000 km²) over the last 50 years.
Reports indicate that the U.S. lost 50% of its East Coast seagrass beds by the mid-1970s primarily due to anthropogenic influences such as agriculture and urbanization.

To combat eutrophication and sedimentation, potential solutions include:
- Implementing strict discharge regulations.
- Employing tertiary sewage treatment methods.
- Reducing fertilizer usage and promoting vegetation buffers around waterways.
- Reducing rates of deforestation and crop monocultures.
Effective management strategies are crucial for fostering recovery in affected ecosystems.

Coastal habitats are universally threatened by eutrophication and sedimentation.
The sensitivity of seagrass and coral reefs to bottom-up processes necessitates urgent attention.
Further evaluation is needed to assess impacts of global declines on ecosystem functions.
Subsequently, restoration efforts can yield positive outcomes if defined management strategies are followed.