Algae, Limnology, and Aquatic Scene Analysis

Macroalgae are the stringy green or brown growths visible on rocks, submerged vegetation, and the margins of ponds, canals, and slow rivers. Unlike microscopic diatoms, macroalgal fragments are visible to the naked eye and may be noticed and collected at scene examination. Under a microscope, their cell morphology and arrangement are genus-diagnostic: Spirogyra has a characteristic spiral chloroplast; Cladophora forms a branching filament; Oedogonium has distinctive ring-shaped cell caps at division sites.

The forensic value of macroalgae lies in habitat specificity at the local level. Spirogyra is a warm, still-water species, absent from fast turbulent rivers. Cladophora tolerates higher current and is common on hard surfaces in rivers and lake shores. Finding filaments from a warm-water genus on clothing found at a supposed river crime scene indicates an inconsistency that warrants investigation.

Cyanobacteria (historically called blue-green algae) form surface scums and dense bottom mats in warm, nutrient-enriched water bodies during summer and early autumn. Their forensic significance comes from three properties. First, they are ecologically specific: large cyanobacterial blooms occur only in eutrophic to hypertrophic standing or slow-moving water. Finding cyanobacterial material on a body argues against a fast-flowing river or oligotrophic upland lake as the drowning environment.

Second, several genera produce morphologically distinctive resistant structures. Microcystis forms spherical or irregular mucilaginous colonies that collapse on drying but retain a characteristic gelatinous appearance. Aphanizomenon forms flat bundles of parallel trichomes identifiable by their shape. Anabaena and Nostoc produce akinetes: thick-walled spore-like cells that survive desiccation and mild fixation and are identifiable under light microscopy even in decomposed material.

A water body's phytoplankton community comprises diatoms plus green algae (chlorophytes), chrysophytes, euglenoids, dinoflagellates, and cryptophytes. Each group responds differently to water chemistry, and the combined assemblage is more distinctive than diatoms alone. This is particularly useful for water bodies that are diatom-poor but have characteristic communities of other groups.

Forensically, phytoplankton analysis is applied most usefully when evidence is filtered rather than digested: a surface filter from the clothing of a body, or a filtration of water recovered from a vehicle's foot well or trunk, may yield intact cells from multiple groups. Acid digestion for diatoms destroys green algae, chrysophytes, and cyanobacterial cells; if those groups are important in the suspected water body, an alternative preparation that preserves them (such as formalin fixation and light microscopy, or flow cytometry for fresh samples) should be considered.

Submerged and emergent aquatic plants (macrophytes) frequently detach fragments during disturbance and adhere to clothing, footwear, skin, and vehicle exteriors. The distribution and species composition of these fragments carry two types of information: habitat type (what kind of water body) and entry geometry (where on the body or vehicle the plant contacted the water).

• Reed (Phragmites australis): dominant emergent plant of lake and river margins globally. Fragments of reed stems, leaves, or seed heads on lower legs and footwear suggest the victim entered the water at a vegetated margin rather than an open-water location such as a bridge.
• Water hyacinth (Eichhornia crassipes): an invasive floating macrophyte in tropical and subtropical water bodies across South Asia, Africa, and South America. Its presence on a body is highly geographically specific and immediately narrows the candidate water bodies in any region where it is not established.
• Pondweeds (Potamogeton spp.): submerged plants common in nutrient-rich slow-flowing water. Leaf fragments and stipule characters allow species-level identification and can distinguish lake-margin and canal habitats from flowing rivers.
• Waterweed (Elodea canadensis): an introduced fully submerged species in many parts of the world, extremely common in ponds, canals, and slow rivers. Its whorled leaf arrangement and cell morphology are highly characteristic and easily identified under a hand lens.

The distribution of plant fragments on a body can also be informative about body position and movement. Fragments concentrated on the back suggest the person floated face-down; fragments on the scalp and face suggest face-up floating or head-first entry. Fragments on the dorsal surface of a vehicle suggest it was driven into or through vegetated water, not simply parked at a bank.

A limnological profile of a candidate water body provides the reference framework for interpreting all aquatic biological evidence. It should include, at a minimum:

• Physical parameters: temperature (surface and depth profile), transparency (Secchi depth), colour, and turbidity. These constrain the biological communities present.
• Chemical parameters: pH, conductivity, dissolved oxygen, total phosphorus, total nitrogen, and chlorophyll-a. These determine trophic status and the dominant biological groups expected.
• Biological inventory: phytoplankton (quantitative plankton net haul and filtered water sample for diatom preparation), macroalgae (shore transect), macrophyte species list (perimeter survey), and if time allows, macroinvertebrate and zooplankton census.
• Spatial sampling: samples from multiple points around the water body (inlet, outlet, margins, open water) because the phytoplankton and macrophyte communities vary within a single lake.

The profile is then compared against the biological material recovered from the evidence. Agreement across multiple parameters and multiple taxonomic groups strengthens the association. Disagreement is equally useful: if the suspect claims to have been at a particular water body but the biological evidence on their clothing matches a different type of environment, that is a significant inconsistency.

The strongest aquatic forensic evidence comes from combining multiple biological groups rather than relying on one. Diatoms provide the best post-mortem persistence (silica survives decomposition); macroalgae and macrophytes provide habitat geometry and entry-point clues; phytoplankton assemblages provide a distinctive multi-taxon fingerprint; and cyanobacteria narrow the trophic context and season.

Each line has vulnerabilities the others partially compensate for. Diatoms are subject to contamination concerns; macroalgal material may be shed or washed away before recovery; cyanobacterial presence is seasonal. When all available lines point to the same water body and the same habitat type, the inferential strength is substantially greater than any single line alone.