Indicators

Introduction

Indicators are measures which define the state of an estuary. The indicators listed below are based on outputs of the SERM model, and are available through the SERM interface. By clicking on each indicator, you can view a description of the indicator, units and plot ranges, and a discussion of its significance in estuarine ecology. In addition, links are provided directly to the SERM explorer page to provide an example of model output for each indicator. These examples use the lagoon estuary type with the full range (or 'all') option selected for all other estuarine parameters.

• annual mean denitrified as % of tn load
• attenuation coefficient
• benthic microalgal biomass
• benthic primary productivity
• chlorophyll a
• denitrification efficiency
• dissolved inorganic nitrogen (DIN)
• dissolved inorganic phosphorus (DIP)
• dissolved oxygen
• macroalgae biomass
• oxygen saturation
• phytoplankton biomass as % total plant biomass
• phytoplankton production as % total production
• residence time
• salinity
• seagrass biomass
• seagrass depth range
• secchi depth
• sediment respiration rate
• total nitrogen
• total phosphorus
• total suspended solids

The indicators provided here are for the most part typical indicators of water quality and estuarine health. Many of these are more or less identical to indicators based on observations, and used in the More Quantitative Assessment component of the Estuarine Audit. The model indicator list includes some which are more difficult to measure, and a number of high level or integrative indicators of system function which are included for interest.

Many of the indicators can take values spanning two or three orders of magnitude, and are plotted by default using log scaled axes in the SERM Interface. To make it easier to compare and contrast plots, we have provided standard colour-coded sub-ranges for each indicator. Where indicators are comparable, we have tried to select ranges similar to those used in the More Quantitative Assessment. However, we have not allowed the SERM Indicator ranges to vary geographically. We suggest that the colour-coded ranges be thought of as delineating “low, medium, high and very high” values, rather than being directly interpreted in terms of estuarine health.

Indicator statistics

At the SERM explorer page, the following options are given for statistics:

Select spatial coverage

Spatial coverage determines the location of model outputs used to determine indicator statistics. This option does not appear if a lagoon is specified, as lagoons have no spatial resolution. For tidal and salt-water estuaries, there are 5 boxes along the estuary, and head, middle and mouth refer to the freshwater end, middle and saltwater end of the estuary respectively. For salt-wedge estuaries the additional terms top and bottom refer to the fresher surface layer and the underlying salt wedge layer respectively.

As well as individual cells, for salt-wedge and tidal estuaries, it is possible to choose statistics for the full length of estuary, which are based on samples from all five cells along the estuary. Because the head / middle / mouth are only three of the five model boxes, statistics for full length of estuary include data from boxes not accessible through the head, middle and mouth statistics. In variables with strong spatial gradients, full length of estuary statistics may reflect spatial rather than temporal variation. As an example, if full length of estuary is chosen, the top 20% of points are likely to be from the mouth box, while the bottom 20% are likely to be from the fresh water box.

Full length of estuary is an important statistic when location is considered to be unimportant, and for assessing whether an indicator has exceeded a particular value for any part of an estuary (e.g. when considering human impacts).

Select annual statistic

At the SERM explorer page (link), the user can choose annual statistics of minimum, maximum and mean, and the 5, 10, 20, 50 (or median), 80, 90 and 95th percentiles. Annual statistics were obtained from sampling the value of the indicator every 5 days (74 points in all) during the 10th (final) year for the spatial range specified by above. Simulations had typically reached a stable seasonal cycle by the 10th year.

For annually averaged indicators (such as annual mean seagrass depth etc.), the indicator is always plotted as a mean, independent of what it is graphed against.

Note that the most relevant statistic for estuarine health may vary, depending on the indicator and the issue. For example, you may wish to look at the 80th percentile chlorophyll concentration as a measure of algal bloom intensity, but the 5th percentile dissolved oxygen concentration may be the most relevant statistic for hypoxia (or low oxygen) events. In the descriptions of the individual indicators, some suggestions are given for relevant statistics for a particular indicator.