What is Lake Merced and why is it being threatened?

Map of Lake Merced

Lake Merced is a freshwater lake that provides habitat for over 200 species of birds, reptiles, amphibians, plants, and fish. It is the only freshwater wetland in San Francisco. The Department of Fish and Game once reported that Lake Merced had one of the richest marine ecosystems of all the lakes in northern California. The water levels were so high that Lake Merced periodically flushed out to the ocean over the streets of Sloat and Vicente. Water quality was exceptional and tourists were able to relax and enjoy the crystal clear waters while fishing in their tiny rowboats, but no more.
The lake is now being threatened by urban pollution and its marine wildlife has decreased tremendously. Runoff from the streets about the lake have turned the once clear waters into a murky green cesspool. Rounds shot off by members of the Pacific Rod and Gun Club have potentially poisoned the lake with lead. The water level of the lake reduced tremendously during 1950-1990. Surface creeks that had once supplied fresh water to the lake have been diverted and the lake itself pumped to make room for the surrounding golf course and gun range. The Westside Groundwater Basin Water Channel has all but stopped due to urbanization. These along with summer droughts has lowered the water level and intensified the issue of pollution.

Here are pictures of the golf coarse and shooting range around Lake Merced:

Here is a graph of the turbidity of the lake taken from 1980-1999 from the website http://www.lmtf.org/FoLM/Data/SFSUReports/water.htm

It shows that the amount of organic and insoluble pollutants in the water of Lake Merced is increasing to a critical level. If left unchecked, it will soon be too late to restore its waters.

Here is a graph of the acidity levels and alkalinity levels of the lake taken from 1960-199:

The data shows that the lake itself has a basic pH which means high alkaline levels. When I personally took samples from Lake Merced and ran them under litmus paper on November 22nd, 2012, I found the pH level to be between 7-8.3 confirming this data.

Here is a graph of the dissolved oxygen levels of the lake at both the surface and 15 feet under the lake from 1997 to 1999:





This shows a slight decrease in the amount of dissolved oxygen in the lake itself. Many marine organisms rely on a stable dissolved oxygen level. Thus, attributing to the gradual disappearance of much of the marine life in Lake Merced. This problem is caused by the rise in turbidity and the basification of the lake.

Here is a graph of the temperature of the lake at the surface from 1997 to 1999 and 15 feet under the lake from 2000 to 2005:



 

Temperature is vital to many biological functions in marine life. The temperature shows an inverse relationship with the amount of dissolved oxygen fifteen feet under the lake. Unfortunately, during cold winter periods, many marine wildlife are not able to function in the frigid waters far below the surface.

Here is a graph of phosphorus and nitrogen levels of the lake from 1997 to 1999:





Phosphorus levels are slightly higher than that of nitrogen ammonia probably because of chemical runoff from the nearby streets and golf course, but both levels are quite stable. This means that algal bloom should not be a very detrimental problem in the lake's foreseeable future.

Here is a graph of the lake level from June 1999 to April 2006

This is the most significant issue. It shows a significant downward trend in the water level of the lake and attests to the increased concentration of pollutants. This was also during the period in which marine life began to disappear. However, the level has increased since 2002 due to an addition of "system water." We are currently maintaining a water level of 21 feet. However, this is not a permanent solution as the environment itself has not produced this level of water.

And here is a comparison of the lake today and its former glory:

As you can clearly see, the clarity of the lake and it's nearby vegetation has deteriorated.

What is a freshwater wetland and what kinds are there in northern California?

Let's start with understanding what a freshwater wetland is.
A freshwater wetland is an aquatic habitat that contains non-saline water.
There are 4 main typed of Freshwater Wetlands in North America: Ponds and Lakes, Marshes, Swamps, and Peatbogs

o   A pond or lake is a well-defined basin that is filled with stagnant water and ringed by vegetation. It is fed mainly by rainstorms and snowmelt, and loses most of its water through seepage and evaporation. In hot, dry months, parts of a pond or lake may dry out, exposing mud flats. In shallow ponds or lakes, bottom rooted plants such as water lilies can reach the surface, while milfoils, pondweeds, and other submergent plants thrive below the surface.

o   A marsh is usually found near a river, lake or tidal waters. Marshes are subject to periodic flooding, and the water level can change drastically in a short amount of time. The boundaries of a marsh are not well defined, and in draught, a marsh can completely dry up. Marshes are overgrown with coarse grasses, sedges and rushes.

o   A swamp is essentially a wooded marsh. Unlike Marshes, Swamps can support trees, tall shrubs, herbs and mosses. Swamps are covered with still or gently flowing water during wet seasons.

o   A peatbog is a poorly drained area that is covered by mats of moss, which slowly decompose in successive layers and eventually form a material called peat. There are 2 types of peatbogs; Bogs and Fens. The water of a Bog is much more acidic than that of a Fen, due to the larger quantities of sphagnum moss and the more advanced process of decomposition in a Bog. The vegetation of a Peatbog is mainly Sedges, Spruce and low-growing members of the Heath family.