Mapping aquatic vegetation (macrophytes) - field methodology

Methods guide · June 27, 2026

On a warm, windless day, when the water is clear, the team spreads out along the bank - someone wades a transect from the waterline towards the depths, someone identifies one species after another, someone estimates how densely they cover the bottom. Mapping macrophytes is patient, attentive work that calls for an eye for plants and a hand for notes. Whoever maps reads the water through its vegetation: which species grow, how densely and how deep.

Macrophytes - aquatic plants - are a biological indicator of the state of waters: which species grow, how abundantly and in what arrangement, says something about a water body’s fertility, clarity and degree of alteration. And because they are usually mapped on large, remote water bodies - often by several people and without signal - it is another study in which good organisation of the team’s data counts for as much as a good eye for species.

What and how we assess

Rivers and lakes are surveyed with different protocols - choose yours before you go, because data collected with one cannot be converted to the other:

• Rivers (MIR): on a 100 m reach you list the species and estimate the cover of each on a nine-degree scale.
• Lakes (ESMI, high-alkalinity lakes): on transects from the shore to the limit of plant occurrence you record the maximum depth of plant occurrence and the cover of the whole transect as a percentage (to an accuracy of about 10 %), and then the plant communities, not species, with the share of each on the seven-degree Braun-Blanquet scale (r, +, 1-5), taken relative to the vegetated area, not the whole transect. The index also needs areas: of the phytolittoral, of the lake and of the zone shallower than 2.5 m.

From these data the macrophyte indices used in assessing ecological status are calculated. A MIR field sheet is not suitable for ESMI. ESMI is not used in lobelia lakes; coastal lakes and lagoons have separate variants.

The cover scale for rivers (MIR) - use the one the index is computed with

The most common mistake at the start is inventing your own scale (“little / medium / a lot”). The MIR methodology uses a nine-degree cover scale, non-linear - and if you ever want to compute the index from these data, record in it from the outset:

Note that the lower degrees are very narrow (they distinguish 0.1% from 1%) while the upper ones are wide. That is deliberate: trophic state is told above all by the presence and small share of indicator species, not by whether the dominant covers 60 or 70% of the bed.

The scale in lakes (ESMI)

In lakes the unit is not the species but the plant community, and you assess its share of the vegetated area on the seven-degree Braun-Blanquet scale. For the calculation, the GIOŚ handbook converts the degrees into mean cover:

What you will need

• an identification key for aquatic plants, possibly equipment for retrieval (rake, grapnel),
• a boat / wading, sometimes an underwater viewer (a box with a glass pane),
• a transect sheet and - with a team - an agreed cover scale (e.g. percentage or a few-step scale).

How to proceed - step by step

1. Designate fixed reaches / transects - in rivers a survey reach 100 m long, in lakes transects at least 30 m wide (50 m in coastal lakes and lagoons), run from the shore to the maximum depth of plant occurrence.
2. In a river, identify the species and estimate the cover of each on the nine-degree scale (above). In a lake, identify the communities and assess their share on the Braun-Blanquet scale.
3. Record the depth limit of the vegetation - in lakes it is not an extra but a component of the index: ESMI uses the ratio of the vegetated area to the area bounded by the 2.5 m isobath.
4. Keep a uniform scale and method across the whole team - otherwise the results are not comparable.
5. From these data, calculate the index: MIR for rivers, ESMI for lakes - each from its own protocol.

What the result means

Rich submerged vegetation, reaching deep, usually goes with clear and less fertile water; the dominance of a few species and a shallow reach - with altered water. The greatest value lies in the trend: the same transects repeated each season show whether the water body is becoming overgrown, impoverished, or recovering.

The most common mistakes

• Different cover scales within the team - inconsistent data.
• Omitting the depth limit - we lose an important signal of clarity.
• Changing transects - you can only compare the same place and method.
• Mapping out of season - the vegetation has to be developed.

When and how often to map

Usually once in the growing season, on the same transects and with the same method - without repeatability there is nothing to compare. The date matters substantively, because the index describes vegetation at full development: in temperate conditions that is usually July-September. Mapping before the peak of the growing season understates cover and changes the index result: in lakes (ESMI) the assessment will usually come out worse than reality, in rivers (MIR) it can shift either way. In lakes GIOŚ recommends July or the first half of August and allows the period from mid-June to mid-September. Record the date together with the result - in cross-season comparisons it is the first thing that explains differences.

Sources

• River mapping methodology and the MIR index: GIOŚ, macrophytes in rivers (PDF) - the source of the cover scale and of the length of the survey reach.
• Field surveys in lakes (transects, phytolittoral, ESMI): GIOŚ - ESMI guidelines (PDF).
• Field surveys in lakes per the current handbook: GIOŚ (2020), A. Kolada (red.), Podręcznik do monitoringu elementów biologicznych i klasyfikacji stanu ekologicznego wód powierzchniowych. Aktualizacja metod, chapter Makrofity w jeziorach, pp. 256-270 (PDF).
• How an assessment is produced from these data: the macrophyte index MIR and ESMI.

How to make it easier

Mapping is multi-point data collected as a team, often without signal: species and cover on many transects, repeated season after season. On paper it quickly turns into a tangle, and merging several people’s notes can take longer than the survey itself.

LimnoLog brings order to it for you:

• Offline mode - you record on the transect without signal, syncing when you are back in range.
• Teamwork - a shared project, invited members and roles; everyone enters into the same transects and sessions, and all see one set of data instead of several spreadsheets.
• Validation - the coordinator reviews and approves the session (consistent scale, completeness) before it goes into the summaries.
• Custom indicators - species, cover and depth limit are recorded like any other indicator; then comes the chart, sharing via a link and export.

Alongside macrophytes you will also record transparency and nutrients there - vegetation and water chemistry come together into one picture of the water body.

Early users get free access for now and keep the core app free for good (add-ons on Founder programme terms). If you map aquatic vegetation - take a look and try it.