Pollution par les nutriments: Enquête sur les herbiers de zostères dans les estuaires marins et les baies côtières du nord et de l'est du N.-B.

Selon plusieurs scientifiques marins, la charge de nutriments ou l'eutrophisation est actuellement le problème le plus répandu de la qualité des eaux des écosystèmes côtiers. 

Les nutriments (l'azote et le phosphore) ont été qualifiés de classe de polluants la plus nuisible aux environnements marins. Éventuellement, la pollution par les nutriments résulte dans une simplification écologique. 

Au cours de juillet 2002, dans le cadre de son engagement continu à s'occuper de la question de la pollution par les nutriments dans les eaux marines du Nouveau-Brunswick, le Conseil de conservation du Nouveau-Brunswick a initié une évaluation de l'eutrophication des herbiers de zostères dans certains estuaires du nord et de l'est du Nouveau-Brunswick.

 

 

 

Glossary*

algae - any of various simple plant organisms that contain chlorophyll (and can therefore carry out photosynthesis) and live in aquatic habitats  

algal bloom -
a relatively high concentration of microalgae (phytoplankton) or macroalgae (seaweeds)

anoxic - environmental condition in which there is no free dissolved oxygen such as anoxic waters or anoxic sediments  

benthic - related to the sea bottom environment  

benthos - plant and animal organisms that live on or in sea bottom sediments  

biochemical oxygen demand (BOD) - the amount of dissolved oxygen required for the bacterial decomposition of organic waste in water  

biomass - the total mass of a species or groups of species within a defined area  

bryozoan - a group of aquatic, mostly marine, invertebrates that live in colonies attached to rocks, seaweeds or shells; individuals making up the colonies are about 1 mm long and they are filter feeders; sometimes called moss animals or sea mats  

chlorophyll - one of two pigments (chlorophyll a and chlorophyll b) responsible for the green colour of most plants  

detritus - non-living organic material consisting of dead particulate and dissolved organic material  

detritivore - an animal that eats detritus  

epifauna
organisms that live on the surface of the seafloor, such as scallops, crabs, lobster and shrimp, in comparison to infauna - organisms that live
in the seafloor  

epiphyte - a plant or animal that grows upon another plant but is not parasitic  

eutrophic - waters that are rich in organic matter and nutrients 

eutrophication - process that increases the rate of supply of organic matter to ecosystems, usually related to nutrient loading  

fauna - all animals in a specified habitat  

filter feeding - a method of feeding (found only in the water) by which an animal moves water past some structure it has that is capable of filtering out particles within a particular size range  

flora - the plant population of a specific environment  

herbivore - an animal that eats plants  

hydroids - small, plant-like animals that grow attached to rocks, shells and seaweeds; usually wrongly labeled as “seaweeds”; belong to the same group of animals as jellyfish, sea anemones and corals  

infauna - bottom-dwelling animals that spend most of their lives buried in sea floor sediments, such as sea worms and clams; also known as endofauna  

inorganic
matter other than plant or animal and not containing a combination of carbon/hydrogen/
oxygen  

microalgae - microscopic plants of the sea (phytoplankton)  

macroalgae
also called seaweeds; includes the larger plants of the sea that grow attached to the bottom; there are three classes of macroalgae: green, brown, and red algae  

organic - describes compounds based on carbon and also containing hydrogen, oxygen, nitrogen and other compounds. Organic waste referred to matter of plant or animals origin such as sewage, manure or leaves. Organic pollution refers to human-made synthetic organic compounds, which can include toxic compounds like DDT and PCBs.  

phytoplankton - microscopic aquatic plants that drift in sunlit surface waters  

plankton - minute plants and animals that drift in the sea and are the basic food source for many marine mammals and commercial fish  

primary production - the rate of production of organic matter of a population; amount of new organic matter produced from inorganic material by organisms using photosynthesis (e.g., phytoplankton, macroalgae)  

productivity - the rate of production of organic matter of a population or ecosystem  

salinity - the measure of the amount of salt in a body of water; can be expressed as parts per thousand (0/00) or practical salinity units (PSU)  

sediment - matter that settles on and builds up on the ocean bottom; comprised of muds, sand, clays, shells and remains of living organisms  

species diversity
the number of species in a region, sometimes weighted by their relative abundance and the evenness of their distribution  

species index - a measure of the species structure in a community such as index of dominance, similarity or diversity  

species richness - the number of species
 in a region  

* Source Material for Glossary

Barnes, R.D. (1987) Invertebrate Zoology. Fifth Edition. Saunders College Publishing. 
893 p.  

Oxford Concise Science Dictionary (1996) Oxford University Press. 794 p.  

Environment Canada (1991) The State of Canada’s Environment - 1991. Minister of Supply and Service Canada.  

Odum, E.P. (1971) Fundamentals of Ecology. W.B. Saunders Company. Philadelphia. 574 p.  

Thorne-Miller, B., and J. Catena (1991) The Living Ocean: understanding and protecting marine biodiversity. Island Press, Washington, DC. 180 p.  

White, L., and F. Johns (1997) Marine Environmental Assessment of the Estuary and Gulf of St. Lawrence . Fisheries and Oceans Canada , Dartmouth, Nova Scotia and Mont-Joli, Quebec, 128 p.

 

Nutrient Pollution: 
A survey of eelgrass beds in estuaries and coastal bays
in northern and eastern New Brunswick

 
Inka Milewski
Conservation Council of New Brunswick Inc. (CCNB)
April 2004

According to many marine scientists, nutrient loading or eutrophication is currently the most widespread water quality problem in coastal ecosystems (Vitousek et al.1997; Howarth et al. 2000). Nutrients (nitrogen and phosphorus) have been called the most damaging class of pollutants in the marine environment (GESAMP 1990). The consequence of overloading coastal water with nitrogen (N) begins with an increased growth of both large (macro-) algae such as sea lettuce and small (micro-) algae or phytoplankton. The death and subsequent decomposition of these plants increase the biochemical oxygen demand (BOD) and thus depletes the oxygen content of the water. As nutrient-loading continues, other symptoms can develop, such as harmful algal blooms, "dead zones", fish kills, some shellfish poisonings, and the loss of seagrass beds and perennial seaweeds. Ultimately, nutrient pollution can result in ecological simplification (see illustration below).

Point and non-point sources of nutrients into estuaries

Point Sources

Non-point Sources

wastewater effluent, both municipal and industrial (e.g. pulp mills) runoff from agriculture (e.g., fertilizers)
septic systems runoff from pasture and range

runoff and leachate from waste disposal sites

urban runoff from unsewered  and sewered areas
runoff and infiltration from animal feed lots runoff from abandoned mines
storm sewer outfall  lawns and golf courses
overflows of combined storm and sanitary sewers atmospheric deposition 
fish hatcheries erosion from logging
finfish and shellfish aquaculture facilities wetland conversion including peat mining
(Adapted from Carpenter et al. 1998) coastal construction and development


There have been no studies done to assess the eutrophication of estuaries along the north and east coasts of New Brunswick. However, nutrient loading and its ecological impacts have received considerable attention in other parts of the world, notably Chesapeake Bay, the Gulf of Mexico, and the Baltic, Black, North, Mediterranean and Caspian seas.

As part of CCNB's ongoing commitment to addressing nutrient pollution issues in New Brunswick's marine waters, the Conservation Council initiated a eutrophication assessment of eelgrass beds in selected estuaries in northern and eastern New Brunswick. Since eelgrass beds or meadows are the dominant sub-tidal marine plant community on the east and northeastern coasts of New Brunswick, this habitat was selected as the focus of the eutrophication assessment. The assessment, the first of its kind in New Brunswick, took place during July of 2002.

Ecological value of eelgrass beds

Increase biological diversity

Increase habitat diversity
Provide nursery and foraging areas for many commercial and non-commercial species
Improve water quality 
Absorb nutrients
Stabilize sediments 
Plays critical role in global carbon & nutrient recycling

Within one week, all bays were visited and one representative study site was selected to assess (1) the structure of eelgrass beds (shoot density, canopy height, total cover), (2) the abundance and diversity of annual algae (epiphytes, free-floating and bottom-growing macroalgae), (3) the abundance and diversity of associated animals (filter feeder, epiphytic animals, herbivores, detritivores, predators), (4) the abundance of phytoplankton in the water column (chlorophyll a concentration), and (5) water characteristics (temperature, salinity). The 10 study sites were grouped into low- and high-impacted sites. (See map)

The extent and variety of human impacts over the last decades did differ between bays, especially with respect to nutrient loading. As a consequence, these bays show different signs and stages of eutrophication. According to their status of nutrient loading and signs of degradation, Cocagne and Bouctouche Bay, Baie Sainte-Anne, and Lamèque were among the high-impacted bays, whereas Kouchibouguac and Kouchibouguacis estuary and lagoon, as well as Tabusintac Bay were low-impacted sites.

Both, low- and high-impacted sites showed similar eelgrass bed structure (shoot density, canopy height), but at high-impacted sites, the meadows were more patchy and not as homogeneous as at low-impacted sites, resulting in overall lower eelgrass cover. High-impacted sites showed two-fold increases in epiphyte load, bottom-growing or drifting algae, and phytoplankton concentration compared with low-impacted sites (Figure 1). These high epiphyte and phytoplankton loads increase light limitation on eelgrass, likely reducing eelgrass productivity because eelgrass is dependent on high water clarity. Annual drifting or bottom-growing algae have a short life span and decompose during summer and fall. High loads of decomposing annual algae contribute to low-oxygen or anoxic conditions, which have negative effects on eelgrass health and survival. During our survey, all high-impacted sites showed signs of anoxic conditions and resulting emissions of toxic hydrogen sulfide.

Figure 1. 
Percent cover of (A) eelgrass, (B) epiphytes on eelgrass leaves separated by annual green, brown and red algae, tube diatoms and fauna (hydroids, bryozoans), (C) bottom-growing or drifting annual algae, and (D) concentration of chlorophyll a. Data are means ± 1 SE (n = 10).

The fauna associated with eelgrass beds also showed clear differences between low- and high-impacted sites. Although high-impacted sites had similar filter feeder abundances, the number of detritivores was 6 times higher, the number of herbivores was 3 times lower, and the number of predators was 10 times lower compared to low-impacted sites (Figure 2). These differences in the animal community show a clear shift from a herbivorous to a detritivorous food chain that utilizes the overabundance of decomposing organic matter. Species richness and diversity of the entire community did not differ between low- and high-impacted sites. However, there were severe shifts in the species composition within the community. At sites with high nutrient loading, red algae and epiphytic animals were replaced by green and brown algae, and herbivores were replaced by detritivores.

Figure 2. 
Abundance of (A) filter feeders, (B) herbivores, (C) detritivores, and (D) predators at the 7 survey sites. Data are mean abundance per site (± 1 SE, n = 10).


The results from our field survey indicate clear signs of eutrophication with strong shifts in the plant and animal communities between sites of low and high nutrient loading. Such shifts in community structure can alter the functioning of the community in the ecosystem. Thus, eelgrass meadows in high-nutrient environments may not perform their natural role and ecological importance as well as they do in low-nutrient environments. But, their role as a filter and buffer in the nutrient cycling of the oceans, as a nursery and spawning ground for commercially important fish and invertebrates, as a sediment stabilizer, and as habitat for a high diversity marine flora and fauna are important ecosystem services that contribute to the well-being of coastal ecosystems and human society.

Based on our survey results, we recommend that mitigation efforts should concentrate on the reduction of point and non-point nutrient loading. Sewage treatment plants and control of municipal, commercial and industrial discharges can reduce nutrient loading through point sources, while restoration of wetlands along the coastline and around agriculture operations can serve as a natural filter and buffer between land and sea, through storing and recycling nutrients, thereby reducing non-point nutrient loading.


Heike Lotze (left), Inka Milewski (middle) & Zsofi Koller (right)
take a break to warm-up at the Kouchibouguacis sampling site

(photo: I. Milewski)

This article is an excerpt from the 60-page, colour publication titled, Nutrient Pollution: a eutrophication survey of eelgrass beds in estuaries and coastal bays in northern and eastern New Brunswick,
by Heike Lotze, Inka Milewski, Boris Worm and Zsofi Koller. It can be purchased for $10 from the Conservation Council of
New Brunswick by calling (506) 458-8747.